| Message ID | 45e8fd67-51fd-7828-fe43-d261d6c33727@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm, futex: Fix shared futex pgoff on shmem huge page | expand |
On Fri, Jun 11, 2021 at 09:31:16PM -0700, Hugh Dickins wrote: > +++ linux/include/linux/pagemap.h 2021-06-11 17:30:28.726720252 -0700 > @@ -516,8 +516,7 @@ static inline struct page *read_mapping_ > } > > /* > - * Get index of the page with in radix-tree > - * (TODO: remove once hugetlb pages will have ->index in PAGE_SIZE) > + * Get index of the page within radix-tree (but not for hugetlb pages). > */ I think the TODO should be retained. It's still something that I intend to do. > static inline pgoff_t page_to_pgoff(struct page *page) > { > - if (unlikely(PageHeadHuge(page))) > - return page->index << compound_order(page); > - > + if (unlikely(PageHuge(page))) { > + extern pgoff_t hugetlb_basepage_index(struct page *page); > + return hugetlb_basepage_index(page); > + } > return page_to_index(page); > } Yes, this change absolutely makes sense. It's just a landmine if it returns the right answer for some tail pages but garbage for other kinds of tail pages. > --- 5.13-rc5/mm/hugetlb.c 2021-06-06 16:57:26.263006733 -0700 > +++ linux/mm/hugetlb.c 2021-06-11 17:30:28.730720276 -0700 > @@ -1588,15 +1588,12 @@ struct address_space *hugetlb_page_mappi > return NULL; > } > > -pgoff_t __basepage_index(struct page *page) > +pgoff_t hugetlb_basepage_index(struct page *page) > { > struct page *page_head = compound_head(page); > pgoff_t index = page_index(page_head); > unsigned long compound_idx; > > - if (!PageHuge(page_head)) > - return page_index(page); > - > if (compound_order(page_head) >= MAX_ORDER) > compound_idx = page_to_pfn(page) - page_to_pfn(page_head); > else > urgh. this trailing bit should be: #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP) compound_idx = page_to_pfn(page) - page_to_pfn(page_head); #else compound_idx = page - page_head; #endif
On Sat, 12 Jun 2021, Matthew Wilcox wrote: > On Fri, Jun 11, 2021 at 09:31:16PM -0700, Hugh Dickins wrote: > > +++ linux/include/linux/pagemap.h 2021-06-11 17:30:28.726720252 -0700 > > @@ -516,8 +516,7 @@ static inline struct page *read_mapping_ > > } > > > > /* > > - * Get index of the page with in radix-tree > > - * (TODO: remove once hugetlb pages will have ->index in PAGE_SIZE) > > + * Get index of the page within radix-tree (but not for hugetlb pages). > > */ > > I think the TODO should be retained. It's still something that I > intend to do. Okay. I did not mean to imply, by removing those TODOs, that they should not be done: just that they were a developer's notes to self, that I found distracting there. I've restored both TODOs (but changed the second to say "hugetlb pages" explicitly, rather than the ambiguous "hugepage"). > > --- 5.13-rc5/mm/hugetlb.c 2021-06-06 16:57:26.263006733 -0700 > > +++ linux/mm/hugetlb.c 2021-06-11 17:30:28.730720276 -0700 > > @@ -1588,15 +1588,12 @@ struct address_space *hugetlb_page_mappi > > return NULL; > > } > > > > -pgoff_t __basepage_index(struct page *page) > > +pgoff_t hugetlb_basepage_index(struct page *page) > > { > > struct page *page_head = compound_head(page); > > pgoff_t index = page_index(page_head); > > unsigned long compound_idx; > > > > - if (!PageHuge(page_head)) > > - return page_index(page); > > - > > if (compound_order(page_head) >= MAX_ORDER) > > compound_idx = page_to_pfn(page) - page_to_pfn(page_head); > > else > > > > urgh. this trailing bit should be: > > #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP) > compound_idx = page_to_pfn(page) - page_to_pfn(page_head); > #else > compound_idx = page - page_head; > #endif I don't see what's wrong with what's there, myself. Unfamiliar territory to me, but mem_map_next() appears to have the same MAX_ORDER expectation. Or perhaps you're just suggesting an optimization. If it were obvious to me, I'd have gladly folded it in; but no, please send your own patch for that, running it by Mike Kravetz and Mike Rapoport and David Hildenbrand, I think. Thanks, v2 follows, Hugh
--- 5.13-rc5/include/linux/hugetlb.h 2021-05-09 17:25:09.278703159 -0700 +++ linux/include/linux/hugetlb.h 2021-06-11 17:30:28.726720252 -0700 @@ -733,17 +733,6 @@ static inline int hstate_index(struct hs return h - hstates; } -pgoff_t __basepage_index(struct page *page); - -/* Return page->index in PAGE_SIZE units */ -static inline pgoff_t basepage_index(struct page *page) -{ - if (!PageCompound(page)) - return page->index; - - return __basepage_index(page); -} - extern int dissolve_free_huge_page(struct page *page); extern int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn); @@ -980,11 +969,6 @@ static inline int hstate_index(struct hs return 0; } -static inline pgoff_t basepage_index(struct page *page) -{ - return page->index; -} - static inline int dissolve_free_huge_page(struct page *page) { return 0; --- 5.13-rc5/include/linux/pagemap.h 2021-05-16 22:49:30.036176843 -0700 +++ linux/include/linux/pagemap.h 2021-06-11 17:30:28.726720252 -0700 @@ -516,8 +516,7 @@ static inline struct page *read_mapping_ } /* - * Get index of the page with in radix-tree - * (TODO: remove once hugetlb pages will have ->index in PAGE_SIZE) + * Get index of the page within radix-tree (but not for hugetlb pages). */ static inline pgoff_t page_to_index(struct page *page) { @@ -536,14 +535,14 @@ static inline pgoff_t page_to_index(stru } /* - * Get the offset in PAGE_SIZE. - * (TODO: hugepage should have ->index in PAGE_SIZE) + * Get the offset in PAGE_SIZE (even for hugetlb pages). */ static inline pgoff_t page_to_pgoff(struct page *page) { - if (unlikely(PageHeadHuge(page))) - return page->index << compound_order(page); - + if (unlikely(PageHuge(page))) { + extern pgoff_t hugetlb_basepage_index(struct page *page); + return hugetlb_basepage_index(page); + } return page_to_index(page); } --- 5.13-rc5/kernel/futex.c 2021-05-09 17:25:09.670705811 -0700 +++ linux/kernel/futex.c 2021-06-11 17:30:28.726720252 -0700 @@ -35,7 +35,6 @@ #include <linux/jhash.h> #include <linux/pagemap.h> #include <linux/syscalls.h> -#include <linux/hugetlb.h> #include <linux/freezer.h> #include <linux/memblock.h> #include <linux/fault-inject.h> @@ -650,7 +649,7 @@ again: key->both.offset |= FUT_OFF_INODE; /* inode-based key */ key->shared.i_seq = get_inode_sequence_number(inode); - key->shared.pgoff = basepage_index(tail); + key->shared.pgoff = page_to_pgoff(tail); rcu_read_unlock(); } --- 5.13-rc5/mm/hugetlb.c 2021-06-06 16:57:26.263006733 -0700 +++ linux/mm/hugetlb.c 2021-06-11 17:30:28.730720276 -0700 @@ -1588,15 +1588,12 @@ struct address_space *hugetlb_page_mappi return NULL; } -pgoff_t __basepage_index(struct page *page) +pgoff_t hugetlb_basepage_index(struct page *page) { struct page *page_head = compound_head(page); pgoff_t index = page_index(page_head); unsigned long compound_idx; - if (!PageHuge(page_head)) - return page_index(page); - if (compound_order(page_head) >= MAX_ORDER) compound_idx = page_to_pfn(page) - page_to_pfn(page_head); else
If more than one futex is placed on a shmem huge page, it can happen that waking the second wakes the first instead, and leaves the second waiting: the key's shared.pgoff is wrong. When 3.11 commit 13d60f4b6ab5 ("futex: Take hugepages into account when generating futex_key"), the only shared huge pages came from hugetlbfs, and the code added to deal with its exceptional page->index was put into hugetlb source. Then that was missed when 4.8 added shmem huge pages. page_to_pgoff() is what others use for this nowadays: except that, as currently written, it gives the right answer on hugetlbfs head, but nonsense on hugetlbfs tails. Fix that by calling hugetlbfs-specific hugetlb_basepage_index() on PageHuge tails as well as on head. Yes, it's unconventional to declare hugetlb_basepage_index() there in pagemap.h, rather than in hugetlb.h; but I do not expect anything but page_to_pgoff() ever to need it. Fixes: 800d8c63b2e9 ("shmem: add huge pages support") Reported-by: Neel Natu <neelnatu@google.com> Signed-off-by: Hugh Dickins <hughd@google.com> Cc: <stable@vger.kernel.org> --- include/linux/hugetlb.h | 16 ---------------- include/linux/pagemap.h | 13 ++++++------- kernel/futex.c | 3 +-- mm/hugetlb.c | 5 +---- 4 files changed, 8 insertions(+), 29 deletions(-)