| Message ID | 20190524153148.18481-1-hannes@cmpxchg.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm: fix page cache convergence regression | expand |
On Fri, May 24, 2019 at 11:31:48AM -0400, Johannes Weiner wrote: > diff --git a/include/linux/xarray.h b/include/linux/xarray.h > index 0e01e6129145..cbbf76e4c973 100644 > --- a/include/linux/xarray.h > +++ b/include/linux/xarray.h > @@ -292,6 +292,7 @@ struct xarray { > spinlock_t xa_lock; > /* private: The rest of the data structure is not to be used directly. */ > gfp_t xa_flags; > + gfp_t xa_gfp; > void __rcu * xa_head; > }; No. I'm willing to go for a xa_flag which says to use __GFP_ACCOUNT, but you can't add another element to the struct xarray. We haven't even finished the discussion from yesterday. I'm going to go back to that thread and keep discussing there.
On Fri, May 24, 2019 at 09:04:17AM -0700, Matthew Wilcox wrote: > On Fri, May 24, 2019 at 11:31:48AM -0400, Johannes Weiner wrote: > > diff --git a/include/linux/xarray.h b/include/linux/xarray.h > > index 0e01e6129145..cbbf76e4c973 100644 > > --- a/include/linux/xarray.h > > +++ b/include/linux/xarray.h > > @@ -292,6 +292,7 @@ struct xarray { > > spinlock_t xa_lock; > > /* private: The rest of the data structure is not to be used directly. */ > > gfp_t xa_flags; > > + gfp_t xa_gfp; > > void __rcu * xa_head; > > }; > > No. I'm willing to go for a xa_flag which says to use __GFP_ACCOUNT, but > you can't add another element to the struct xarray. Ok, we can generalize per-tree gfp flags later if necessary. Below is the updated fix that uses an XA_FLAGS_ACCOUNT flag instead. --- From 63a0dbc571ff38f7c072c62d6bc28192debe37ac Mon Sep 17 00:00:00 2001 From: Johannes Weiner <hannes@cmpxchg.org> Date: Fri, 24 May 2019 10:12:46 -0400 Subject: [PATCH] mm: fix page cache convergence regression Since a28334862993 ("page cache: Finish XArray conversion"), on most major Linux distributions, the page cache doesn't correctly transition when the hot data set is changing, and leaves the new pages thrashing indefinitely instead of kicking out the cold ones. On a freshly booted, freshly ssh'd into virtual machine with 1G RAM running stock Arch Linux: [root@ham ~]# ./reclaimtest.sh + dd of=workingset-a bs=1M count=0 seek=600 + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + ./mincore workingset-a 153600/153600 workingset-a + dd of=workingset-b bs=1M count=0 seek=600 + cat workingset-b + cat workingset-b + cat workingset-b + cat workingset-b + ./mincore workingset-a workingset-b 104029/153600 workingset-a 120086/153600 workingset-b + cat workingset-b + cat workingset-b + cat workingset-b + cat workingset-b + ./mincore workingset-a workingset-b 104029/153600 workingset-a 120268/153600 workingset-b workingset-b is a 600M file on a 1G host that is otherwise entirely idle. No matter how often it's being accessed, it won't get cached. While investigating, I noticed that the non-resident information gets aggressively reclaimed - /proc/vmstat::workingset_nodereclaim. This is a problem because a workingset transition like this relies on the non-resident information tracked in the page cache tree of evicted file ranges: when the cache faults are refaults of recently evicted cache, we challenge the existing active set, and that allows a new workingset to establish itself. Tracing the shrinker that maintains this memory revealed that all page cache tree nodes were allocated to the root cgroup. This is a problem, because 1) the shrinker sizes the amount of non-resident information it keeps to the size of the cgroup's other memory and 2) on most major Linux distributions, only kernel threads live in the root cgroup and everything else gets put into services or session groups: [root@ham ~]# cat /proc/self/cgroup 0::/user.slice/user-0.slice/session-c1.scope As a result, we basically maintain no non-resident information for the workloads running on the system, thus breaking the caching algorithm. Looking through the code, I found the culprit in the above-mentioned patch: when switching from the radix tree to xarray, it dropped the __GFP_ACCOUNT flag from the tree node allocations - the flag that makes sure the allocated memory gets charged to and tracked by the cgroup of the calling process - in this case, the one doing the fault. To fix this, allow xarray users to specify per-tree flag that makes xarray allocate nodes using __GFP_ACCOUNT. Then restore the page cache tree annotation to request such cgroup tracking for the cache nodes. With this patch applied, the page cache correctly converges on new workingsets again after just a few iterations: [root@ham ~]# ./reclaimtest.sh + dd of=workingset-a bs=1M count=0 seek=600 + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + ./mincore workingset-a 153600/153600 workingset-a + dd of=workingset-b bs=1M count=0 seek=600 + cat workingset-b + ./mincore workingset-a workingset-b 124607/153600 workingset-a 87876/153600 workingset-b + cat workingset-b + ./mincore workingset-a workingset-b 81313/153600 workingset-a 133321/153600 workingset-b + cat workingset-b + ./mincore workingset-a workingset-b 63036/153600 workingset-a 153600/153600 workingset-b Cc: stable@vger.kernel.org # 4.20+ Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> --- fs/inode.c | 2 +- include/linux/xarray.h | 1 + lib/xarray.c | 12 ++++++++++-- 3 files changed, 12 insertions(+), 3 deletions(-) diff --git a/fs/inode.c b/fs/inode.c index e9d18b2c3f91..cd67859dbaf1 100644 --- a/fs/inode.c +++ b/fs/inode.c @@ -361,7 +361,7 @@ EXPORT_SYMBOL(inc_nlink); static void __address_space_init_once(struct address_space *mapping) { - xa_init_flags(&mapping->i_pages, XA_FLAGS_LOCK_IRQ); + xa_init_flags(&mapping->i_pages, XA_FLAGS_LOCK_IRQ | XA_FLAGS_ACCOUNT); init_rwsem(&mapping->i_mmap_rwsem); INIT_LIST_HEAD(&mapping->private_list); spin_lock_init(&mapping->private_lock); diff --git a/include/linux/xarray.h b/include/linux/xarray.h index 0e01e6129145..5921599b6dc4 100644 --- a/include/linux/xarray.h +++ b/include/linux/xarray.h @@ -265,6 +265,7 @@ enum xa_lock_type { #define XA_FLAGS_TRACK_FREE ((__force gfp_t)4U) #define XA_FLAGS_ZERO_BUSY ((__force gfp_t)8U) #define XA_FLAGS_ALLOC_WRAPPED ((__force gfp_t)16U) +#define XA_FLAGS_ACCOUNT ((__force gfp_t)32U) #define XA_FLAGS_MARK(mark) ((__force gfp_t)((1U << __GFP_BITS_SHIFT) << \ (__force unsigned)(mark))) diff --git a/lib/xarray.c b/lib/xarray.c index 6be3acbb861f..446b956c9188 100644 --- a/lib/xarray.c +++ b/lib/xarray.c @@ -298,6 +298,8 @@ bool xas_nomem(struct xa_state *xas, gfp_t gfp) xas_destroy(xas); return false; } + if (xas->xa->xa_flags & XA_FLAGS_ACCOUNT) + gfp |= __GFP_ACCOUNT; xas->xa_alloc = kmem_cache_alloc(radix_tree_node_cachep, gfp); if (!xas->xa_alloc) return false; @@ -325,6 +327,8 @@ static bool __xas_nomem(struct xa_state *xas, gfp_t gfp) xas_destroy(xas); return false; } + if (xas->xa->xa_flags & XA_FLAGS_ACCOUNT) + gfp |= __GFP_ACCOUNT; if (gfpflags_allow_blocking(gfp)) { xas_unlock_type(xas, lock_type); xas->xa_alloc = kmem_cache_alloc(radix_tree_node_cachep, gfp); @@ -358,8 +362,12 @@ static void *xas_alloc(struct xa_state *xas, unsigned int shift) if (node) { xas->xa_alloc = NULL; } else { - node = kmem_cache_alloc(radix_tree_node_cachep, - GFP_NOWAIT | __GFP_NOWARN); + gfp_t gfp = GFP_NOWAIT | __GFP_NOWARN; + + if (xas->xa->xa_flags & XA_FLAGS_ACCOUNT) + gfp |= __GFP_ACCOUNT; + + node = kmem_cache_alloc(radix_tree_node_cachep, gfp); if (!node) { xas_set_err(xas, -ENOMEM); return NULL;
On Fri, May 24, 2019 at 10:41 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > On Fri, May 24, 2019 at 09:04:17AM -0700, Matthew Wilcox wrote: > > On Fri, May 24, 2019 at 11:31:48AM -0400, Johannes Weiner wrote: > > > diff --git a/include/linux/xarray.h b/include/linux/xarray.h > > > index 0e01e6129145..cbbf76e4c973 100644 > > > --- a/include/linux/xarray.h > > > +++ b/include/linux/xarray.h > > > @@ -292,6 +292,7 @@ struct xarray { > > > spinlock_t xa_lock; > > > /* private: The rest of the data structure is not to be used directly. */ > > > gfp_t xa_flags; > > > + gfp_t xa_gfp; > > > void __rcu * xa_head; > > > }; > > > > No. I'm willing to go for a xa_flag which says to use __GFP_ACCOUNT, but > > you can't add another element to the struct xarray. > > Ok, we can generalize per-tree gfp flags later if necessary. > > Below is the updated fix that uses an XA_FLAGS_ACCOUNT flag instead. > > --- > From 63a0dbc571ff38f7c072c62d6bc28192debe37ac Mon Sep 17 00:00:00 2001 > From: Johannes Weiner <hannes@cmpxchg.org> > Date: Fri, 24 May 2019 10:12:46 -0400 > Subject: [PATCH] mm: fix page cache convergence regression > > Since a28334862993 ("page cache: Finish XArray conversion"), on most > major Linux distributions, the page cache doesn't correctly transition > when the hot data set is changing, and leaves the new pages thrashing > indefinitely instead of kicking out the cold ones. > > On a freshly booted, freshly ssh'd into virtual machine with 1G RAM > running stock Arch Linux: > > [root@ham ~]# ./reclaimtest.sh > + dd of=workingset-a bs=1M count=0 seek=600 > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + ./mincore workingset-a > 153600/153600 workingset-a > + dd of=workingset-b bs=1M count=0 seek=600 > + cat workingset-b > + cat workingset-b > + cat workingset-b > + cat workingset-b > + ./mincore workingset-a workingset-b > 104029/153600 workingset-a > 120086/153600 workingset-b > + cat workingset-b > + cat workingset-b > + cat workingset-b > + cat workingset-b > + ./mincore workingset-a workingset-b > 104029/153600 workingset-a > 120268/153600 workingset-b > > workingset-b is a 600M file on a 1G host that is otherwise entirely > idle. No matter how often it's being accessed, it won't get cached. > > While investigating, I noticed that the non-resident information gets > aggressively reclaimed - /proc/vmstat::workingset_nodereclaim. This is > a problem because a workingset transition like this relies on the > non-resident information tracked in the page cache tree of evicted > file ranges: when the cache faults are refaults of recently evicted > cache, we challenge the existing active set, and that allows a new > workingset to establish itself. > > Tracing the shrinker that maintains this memory revealed that all page > cache tree nodes were allocated to the root cgroup. This is a problem, > because 1) the shrinker sizes the amount of non-resident information > it keeps to the size of the cgroup's other memory and 2) on most major > Linux distributions, only kernel threads live in the root cgroup and > everything else gets put into services or session groups: > > [root@ham ~]# cat /proc/self/cgroup > 0::/user.slice/user-0.slice/session-c1.scope > > As a result, we basically maintain no non-resident information for the > workloads running on the system, thus breaking the caching algorithm. > > Looking through the code, I found the culprit in the above-mentioned > patch: when switching from the radix tree to xarray, it dropped the > __GFP_ACCOUNT flag from the tree node allocations - the flag that > makes sure the allocated memory gets charged to and tracked by the > cgroup of the calling process - in this case, the one doing the fault. > > To fix this, allow xarray users to specify per-tree flag that makes > xarray allocate nodes using __GFP_ACCOUNT. Then restore the page cache > tree annotation to request such cgroup tracking for the cache nodes. > > With this patch applied, the page cache correctly converges on new > workingsets again after just a few iterations: > > [root@ham ~]# ./reclaimtest.sh > + dd of=workingset-a bs=1M count=0 seek=600 > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + ./mincore workingset-a > 153600/153600 workingset-a > + dd of=workingset-b bs=1M count=0 seek=600 > + cat workingset-b > + ./mincore workingset-a workingset-b > 124607/153600 workingset-a > 87876/153600 workingset-b > + cat workingset-b > + ./mincore workingset-a workingset-b > 81313/153600 workingset-a > 133321/153600 workingset-b > + cat workingset-b > + ./mincore workingset-a workingset-b > 63036/153600 workingset-a > 153600/153600 workingset-b > > Cc: stable@vger.kernel.org # 4.20+ > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Reviewed-by: Shakeel Butt <shakeelb@google.com>
Are there any objections or feedback on the proposed fix below? This is kind of a serious regression. On Fri, May 24, 2019 at 01:39:00PM -0400, Johannes Weiner wrote: > From 63a0dbc571ff38f7c072c62d6bc28192debe37ac Mon Sep 17 00:00:00 2001 > From: Johannes Weiner <hannes@cmpxchg.org> > Date: Fri, 24 May 2019 10:12:46 -0400 > Subject: [PATCH] mm: fix page cache convergence regression > > Since a28334862993 ("page cache: Finish XArray conversion"), on most > major Linux distributions, the page cache doesn't correctly transition > when the hot data set is changing, and leaves the new pages thrashing > indefinitely instead of kicking out the cold ones. > > On a freshly booted, freshly ssh'd into virtual machine with 1G RAM > running stock Arch Linux: > > [root@ham ~]# ./reclaimtest.sh > + dd of=workingset-a bs=1M count=0 seek=600 > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + ./mincore workingset-a > 153600/153600 workingset-a > + dd of=workingset-b bs=1M count=0 seek=600 > + cat workingset-b > + cat workingset-b > + cat workingset-b > + cat workingset-b > + ./mincore workingset-a workingset-b > 104029/153600 workingset-a > 120086/153600 workingset-b > + cat workingset-b > + cat workingset-b > + cat workingset-b > + cat workingset-b > + ./mincore workingset-a workingset-b > 104029/153600 workingset-a > 120268/153600 workingset-b > > workingset-b is a 600M file on a 1G host that is otherwise entirely > idle. No matter how often it's being accessed, it won't get cached. > > While investigating, I noticed that the non-resident information gets > aggressively reclaimed - /proc/vmstat::workingset_nodereclaim. This is > a problem because a workingset transition like this relies on the > non-resident information tracked in the page cache tree of evicted > file ranges: when the cache faults are refaults of recently evicted > cache, we challenge the existing active set, and that allows a new > workingset to establish itself. > > Tracing the shrinker that maintains this memory revealed that all page > cache tree nodes were allocated to the root cgroup. This is a problem, > because 1) the shrinker sizes the amount of non-resident information > it keeps to the size of the cgroup's other memory and 2) on most major > Linux distributions, only kernel threads live in the root cgroup and > everything else gets put into services or session groups: > > [root@ham ~]# cat /proc/self/cgroup > 0::/user.slice/user-0.slice/session-c1.scope > > As a result, we basically maintain no non-resident information for the > workloads running on the system, thus breaking the caching algorithm. > > Looking through the code, I found the culprit in the above-mentioned > patch: when switching from the radix tree to xarray, it dropped the > __GFP_ACCOUNT flag from the tree node allocations - the flag that > makes sure the allocated memory gets charged to and tracked by the > cgroup of the calling process - in this case, the one doing the fault. > > To fix this, allow xarray users to specify per-tree flag that makes > xarray allocate nodes using __GFP_ACCOUNT. Then restore the page cache > tree annotation to request such cgroup tracking for the cache nodes. > > With this patch applied, the page cache correctly converges on new > workingsets again after just a few iterations: > > [root@ham ~]# ./reclaimtest.sh > + dd of=workingset-a bs=1M count=0 seek=600 > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + cat workingset-a > + ./mincore workingset-a > 153600/153600 workingset-a > + dd of=workingset-b bs=1M count=0 seek=600 > + cat workingset-b > + ./mincore workingset-a workingset-b > 124607/153600 workingset-a > 87876/153600 workingset-b > + cat workingset-b > + ./mincore workingset-a workingset-b > 81313/153600 workingset-a > 133321/153600 workingset-b > + cat workingset-b > + ./mincore workingset-a workingset-b > 63036/153600 workingset-a > 153600/153600 workingset-b > > Cc: stable@vger.kernel.org # 4.20+ > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> > --- > fs/inode.c | 2 +- > include/linux/xarray.h | 1 + > lib/xarray.c | 12 ++++++++++-- > 3 files changed, 12 insertions(+), 3 deletions(-) > > diff --git a/fs/inode.c b/fs/inode.c > index e9d18b2c3f91..cd67859dbaf1 100644 > --- a/fs/inode.c > +++ b/fs/inode.c > @@ -361,7 +361,7 @@ EXPORT_SYMBOL(inc_nlink); > > static void __address_space_init_once(struct address_space *mapping) > { > - xa_init_flags(&mapping->i_pages, XA_FLAGS_LOCK_IRQ); > + xa_init_flags(&mapping->i_pages, XA_FLAGS_LOCK_IRQ | XA_FLAGS_ACCOUNT); > init_rwsem(&mapping->i_mmap_rwsem); > INIT_LIST_HEAD(&mapping->private_list); > spin_lock_init(&mapping->private_lock); > diff --git a/include/linux/xarray.h b/include/linux/xarray.h > index 0e01e6129145..5921599b6dc4 100644 > --- a/include/linux/xarray.h > +++ b/include/linux/xarray.h > @@ -265,6 +265,7 @@ enum xa_lock_type { > #define XA_FLAGS_TRACK_FREE ((__force gfp_t)4U) > #define XA_FLAGS_ZERO_BUSY ((__force gfp_t)8U) > #define XA_FLAGS_ALLOC_WRAPPED ((__force gfp_t)16U) > +#define XA_FLAGS_ACCOUNT ((__force gfp_t)32U) > #define XA_FLAGS_MARK(mark) ((__force gfp_t)((1U << __GFP_BITS_SHIFT) << \ > (__force unsigned)(mark))) > > diff --git a/lib/xarray.c b/lib/xarray.c > index 6be3acbb861f..446b956c9188 100644 > --- a/lib/xarray.c > +++ b/lib/xarray.c > @@ -298,6 +298,8 @@ bool xas_nomem(struct xa_state *xas, gfp_t gfp) > xas_destroy(xas); > return false; > } > + if (xas->xa->xa_flags & XA_FLAGS_ACCOUNT) > + gfp |= __GFP_ACCOUNT; > xas->xa_alloc = kmem_cache_alloc(radix_tree_node_cachep, gfp); > if (!xas->xa_alloc) > return false; > @@ -325,6 +327,8 @@ static bool __xas_nomem(struct xa_state *xas, gfp_t gfp) > xas_destroy(xas); > return false; > } > + if (xas->xa->xa_flags & XA_FLAGS_ACCOUNT) > + gfp |= __GFP_ACCOUNT; > if (gfpflags_allow_blocking(gfp)) { > xas_unlock_type(xas, lock_type); > xas->xa_alloc = kmem_cache_alloc(radix_tree_node_cachep, gfp); > @@ -358,8 +362,12 @@ static void *xas_alloc(struct xa_state *xas, unsigned int shift) > if (node) { > xas->xa_alloc = NULL; > } else { > - node = kmem_cache_alloc(radix_tree_node_cachep, > - GFP_NOWAIT | __GFP_NOWARN); > + gfp_t gfp = GFP_NOWAIT | __GFP_NOWARN; > + > + if (xas->xa->xa_flags & XA_FLAGS_ACCOUNT) > + gfp |= __GFP_ACCOUNT; > + > + node = kmem_cache_alloc(radix_tree_node_cachep, gfp); > if (!node) { > xas_set_err(xas, -ENOMEM); > return NULL; > -- > 2.21.0 >
On Thu, May 30, 2019 at 12:15:48PM -0400, Johannes Weiner wrote: > Are there any objections or feedback on the proposed fix below? This > is kind of a serious regression. I'll drop it into the xarray tree for merging in a week, if that's ok with you?
On Thu, May 30, 2019 at 10:13:56AM -0700, Matthew Wilcox wrote: > On Thu, May 30, 2019 at 12:15:48PM -0400, Johannes Weiner wrote: > > Are there any objections or feedback on the proposed fix below? This > > is kind of a serious regression. > > I'll drop it into the xarray tree for merging in a week, if that's ok > with you? That sounds great, thank you.
On Thu, May 30, 2019 at 10:13:56AM -0700, Matthew Wilcox wrote: > On Thu, May 30, 2019 at 12:15:48PM -0400, Johannes Weiner wrote: > > Are there any objections or feedback on the proposed fix below? This > > is kind of a serious regression. > > I'll drop it into the xarray tree for merging in a week, if that's ok > with you? Hey, it's three weeks later and we're about to miss 5.2. This sucks, Matthew. You introduced a serious regression to the MM subsystem, whose process and patch routing you largely bypassed. When I encountered the problem and provided a reproducer and a fix, you gave me a hard time on cosmetic grounds. I incorporated all your feedback, and still you show no urgency to get this patch or a fix of your own into mainline. It's your bug, please fix it.
On Mon, Jun 24, 2019 at 11:19 PM Johannes Weiner <hannes@cmpxchg.org> wrote: > > Hey, it's three weeks later and we're about to miss 5.2. > > This sucks, Matthew. Yes. And I do think that having a real gfp field there would be better than the very odd xa_flags that is *marked* as being gfp_t, but isn't really a gfp_t at all. So how about we apply Johannes' patch, and then work on making that xa_flags field be a proper type of its own. Because it really isn't a gfp_t, and never has been, even if there might be some very limited and hacky overlap right now. Alternatrively, the subset of bits that _can_ be used as a gfp should actually be used as such, in xas_alloc/xas_nomem. So that you can do xa_init_flags(&mapping->i_pages, XA_FLAGS_LOCK_IRQ | __GFP_ACCOUNT); in __address_space_init_once() and it would do what it is supposed to do. Willy? Linus
diff --git a/fs/inode.c b/fs/inode.c index e9d18b2c3f91..3b454d2119c4 100644 --- a/fs/inode.c +++ b/fs/inode.c @@ -362,6 +362,7 @@ EXPORT_SYMBOL(inc_nlink); static void __address_space_init_once(struct address_space *mapping) { xa_init_flags(&mapping->i_pages, XA_FLAGS_LOCK_IRQ); + mapping->i_pages.xa_gfp = __GFP_ACCOUNT; init_rwsem(&mapping->i_mmap_rwsem); INIT_LIST_HEAD(&mapping->private_list); spin_lock_init(&mapping->private_lock); diff --git a/include/linux/xarray.h b/include/linux/xarray.h index 0e01e6129145..cbbf76e4c973 100644 --- a/include/linux/xarray.h +++ b/include/linux/xarray.h @@ -292,6 +292,7 @@ struct xarray { spinlock_t xa_lock; /* private: The rest of the data structure is not to be used directly. */ gfp_t xa_flags; + gfp_t xa_gfp; void __rcu * xa_head; }; @@ -374,6 +375,7 @@ static inline void xa_init_flags(struct xarray *xa, gfp_t flags) { spin_lock_init(&xa->xa_lock); xa->xa_flags = flags; + xa->xa_gfp = 0; xa->xa_head = NULL; } diff --git a/lib/xarray.c b/lib/xarray.c index 6be3acbb861f..324be9534861 100644 --- a/lib/xarray.c +++ b/lib/xarray.c @@ -298,6 +298,7 @@ bool xas_nomem(struct xa_state *xas, gfp_t gfp) xas_destroy(xas); return false; } + gfp |= xas->xa->xa_gfp; xas->xa_alloc = kmem_cache_alloc(radix_tree_node_cachep, gfp); if (!xas->xa_alloc) return false; @@ -325,6 +326,7 @@ static bool __xas_nomem(struct xa_state *xas, gfp_t gfp) xas_destroy(xas); return false; } + gfp |= xas->xa->xa_gfp; if (gfpflags_allow_blocking(gfp)) { xas_unlock_type(xas, lock_type); xas->xa_alloc = kmem_cache_alloc(radix_tree_node_cachep, gfp); @@ -358,8 +360,10 @@ static void *xas_alloc(struct xa_state *xas, unsigned int shift) if (node) { xas->xa_alloc = NULL; } else { - node = kmem_cache_alloc(radix_tree_node_cachep, - GFP_NOWAIT | __GFP_NOWARN); + gfp_t gfp; + + gfp = GFP_NOWAIT | __GFP_NOWARN | xas->xa->xa_gfp; + node = kmem_cache_alloc(radix_tree_node_cachep, gfp); if (!node) { xas_set_err(xas, -ENOMEM); return NULL;
Since a28334862993 ("page cache: Finish XArray conversion"), on most major Linux distributions, the page cache doesn't correctly transition when the hot data set is changing, and leaves the new pages thrashing indefinitely instead of kicking out the cold ones. On a freshly booted, freshly ssh'd into virtual machine with 1G RAM running stock Arch Linux: [root@ham ~]# ./reclaimtest.sh + dd of=workingset-a bs=1M count=0 seek=600 + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + ./mincore workingset-a 153600/153600 workingset-a + dd of=workingset-b bs=1M count=0 seek=600 + cat workingset-b + cat workingset-b + cat workingset-b + cat workingset-b + ./mincore workingset-a workingset-b 104029/153600 workingset-a 120086/153600 workingset-b + cat workingset-b + cat workingset-b + cat workingset-b + cat workingset-b + ./mincore workingset-a workingset-b 104029/153600 workingset-a 120268/153600 workingset-b workingset-b is a 600M file on a 1G host that is otherwise entirely idle. No matter how often it's being accessed, it won't get cached. While investigating, I noticed that the non-resident information gets aggressively reclaimed - /proc/vmstat::workingset_nodereclaim. This is a problem because a workingset transition like this relies on the non-resident information tracked in the page cache tree of evicted file ranges: when the cache faults are refaults of recently evicted cache, we challenge the existing active set, and that allows a new workingset to establish itself. Tracing the shrinker that maintains this memory revealed that all page cache tree nodes were allocated to the root cgroup. This is a problem, because 1) the shrinker sizes the amount of non-resident information it keeps to the size of the cgroup's other memory and 2) on most major Linux distributions, only kernel threads live in the root cgroup and everything else gets put into services or session groups: [root@ham ~]# cat /proc/self/cgroup 0::/user.slice/user-0.slice/session-c1.scope As a result, we basically maintain no non-resident information for the workloads running on the system, thus breaking the caching algorithm. Looking through the code, I found the culprit in the above-mentioned patch: when switching from the radix tree to xarray, it dropped the __GFP_ACCOUNT flag from the tree node allocations - the flag that makes sure the allocated memory gets charged to and tracked by the cgroup of the calling process - in this case, the one doing the fault. To fix this, allow xarray users to specify per-tree gfp flags that supplement the hardcoded gfp flags inside the xarray expansion code. This is analogous to the radix tree API. Then restore the page cache tree annotation that passes the __GFP_ACCOUNT flag during expansions. With this patch applied, the page cache correctly converges on new workingsets again after just a few iterations: [root@ham ~]# ./reclaimtest.sh + dd of=workingset-a bs=1M count=0 seek=600 + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + cat workingset-a + ./mincore workingset-a 153600/153600 workingset-a + dd of=workingset-b bs=1M count=0 seek=600 + cat workingset-b + ./mincore workingset-a workingset-b 124607/153600 workingset-a 87876/153600 workingset-b + cat workingset-b + ./mincore workingset-a workingset-b 81313/153600 workingset-a 133321/153600 workingset-b + cat workingset-b + ./mincore workingset-a workingset-b 63036/153600 workingset-a 153600/153600 workingset-b Cc: stable@vger.kernel.org # 4.20+ Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> --- fs/inode.c | 1 + include/linux/xarray.h | 2 ++ lib/xarray.c | 8 ++++++-- 3 files changed, 9 insertions(+), 2 deletions(-)