| Message ID | 20220203093232.572380-5-jhubbard@nvidia.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/gup: some cleanups | expand |
On Thu, 3 Feb 2022 01:32:32 -0800 John Hubbard <jhubbard@nvidia.com> wrote: > There is only one caller of get_user_pages_locked(). The purpose of > get_user_pages_locked() is to allow for unlocking the mmap_lock when > reading a page from the disk during a page fault (hidden behind > VM_FAULT_RETRY). The idea is to reduce contention on the heavily-used > mmap_lock. (Thanks to Jan Kara for clearly pointing that out, and in > fact I've used some of his wording here.) > > However, it is unlikely for lookup_node() to take a page fault. With > that in mind, change over to calling get_user_pages_fast(). This > simplifies the code, runs a little faster in the expected case, and > allows removing get_user_pages_locked() entirely. > > Reviewed-by: Jan Kara <jack@suse.cz> > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> > Signed-off-by: John Hubbard <jhubbard@nvidia.com> I have always disliked these functions that might or might not unlock the lock under the hood. I'm happy to see one more go. Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com> > --- > include/linux/mm.h | 2 -- > mm/gup.c | 59 ---------------------------------------------- > mm/mempolicy.c | 21 +++++++---------- > 3 files changed, 9 insertions(+), 73 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 80c540c17d83..528ef1cb4f3a 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1916,8 +1916,6 @@ long get_user_pages(unsigned long start, unsigned long nr_pages, > long pin_user_pages(unsigned long start, unsigned long nr_pages, > unsigned int gup_flags, struct page **pages, > struct vm_area_struct **vmas); > -long get_user_pages_locked(unsigned long start, unsigned long nr_pages, > - unsigned int gup_flags, struct page **pages, int *locked); > long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages, > struct page **pages, unsigned int gup_flags); > long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages, > diff --git a/mm/gup.c b/mm/gup.c > index b0ecbfe03928..7da49df59110 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2118,65 +2118,6 @@ long get_user_pages(unsigned long start, unsigned long nr_pages, > } > EXPORT_SYMBOL(get_user_pages); > > -/** > - * get_user_pages_locked() - variant of get_user_pages() > - * > - * @start: starting user address > - * @nr_pages: number of pages from start to pin > - * @gup_flags: flags modifying lookup behaviour > - * @pages: array that receives pointers to the pages pinned. > - * Should be at least nr_pages long. Or NULL, if caller > - * only intends to ensure the pages are faulted in. > - * @locked: pointer to lock flag indicating whether lock is held and > - * subsequently whether VM_FAULT_RETRY functionality can be > - * utilised. Lock must initially be held. > - * > - * It is suitable to replace the form: > - * > - * mmap_read_lock(mm); > - * do_something() > - * get_user_pages(mm, ..., pages, NULL); > - * mmap_read_unlock(mm); > - * > - * to: > - * > - * int locked = 1; > - * mmap_read_lock(mm); > - * do_something() > - * get_user_pages_locked(mm, ..., pages, &locked); > - * if (locked) > - * mmap_read_unlock(mm); > - * > - * We can leverage the VM_FAULT_RETRY functionality in the page fault > - * paths better by using either get_user_pages_locked() or > - * get_user_pages_unlocked(). > - * > - */ > -long get_user_pages_locked(unsigned long start, unsigned long nr_pages, > - unsigned int gup_flags, struct page **pages, > - int *locked) > -{ > - /* > - * FIXME: Current FOLL_LONGTERM behavior is incompatible with > - * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on > - * vmas. As there are no users of this flag in this call we simply > - * disallow this option for now. > - */ > - if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM)) > - return -EINVAL; > - /* > - * FOLL_PIN must only be set internally by the pin_user_pages*() APIs, > - * never directly by the caller, so enforce that: > - */ > - if (WARN_ON_ONCE(gup_flags & FOLL_PIN)) > - return -EINVAL; > - > - return __get_user_pages_locked(current->mm, start, nr_pages, > - pages, NULL, locked, > - gup_flags | FOLL_TOUCH); > -} > -EXPORT_SYMBOL(get_user_pages_locked); > - > /* > * get_user_pages_unlocked() is suitable to replace the form: > * > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 028e8dd82b44..3f8dc58da3e8 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -907,17 +907,14 @@ static void get_policy_nodemask(struct mempolicy *p, nodemask_t *nodes) > static int lookup_node(struct mm_struct *mm, unsigned long addr) > { > struct page *p = NULL; > - int err; > + int ret; > > - int locked = 1; > - err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked); > - if (err > 0) { > - err = page_to_nid(p); > + ret = get_user_pages_fast(addr & PAGE_MASK, 1, 0, &p); > + if (ret > 0) { > + ret = page_to_nid(p); > put_page(p); > } > - if (locked) > - mmap_read_unlock(mm); > - return err; > + return ret; > } > > /* Retrieve NUMA policy */ > @@ -968,14 +965,14 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask, > if (flags & MPOL_F_NODE) { > if (flags & MPOL_F_ADDR) { > /* > - * Take a refcount on the mpol, lookup_node() > - * will drop the mmap_lock, so after calling > - * lookup_node() only "pol" remains valid, "vma" > - * is stale. > + * Take a refcount on the mpol, because we are about to > + * drop the mmap_lock, after which only "pol" remains > + * valid, "vma" is stale. > */ > pol_refcount = pol; > vma = NULL; > mpol_get(pol); > + mmap_read_unlock(mm); > err = lookup_node(mm, addr); > if (err < 0) > goto out;
On Thu, Feb 03, 2022 at 01:32:32AM -0800, John Hubbard wrote: > There is only one caller of get_user_pages_locked(). The purpose of > get_user_pages_locked() is to allow for unlocking the mmap_lock when > reading a page from the disk during a page fault (hidden behind > VM_FAULT_RETRY). The idea is to reduce contention on the heavily-used > mmap_lock. (Thanks to Jan Kara for clearly pointing that out, and in > fact I've used some of his wording here.) > > However, it is unlikely for lookup_node() to take a page fault. With > that in mind, change over to calling get_user_pages_fast(). This > simplifies the code, runs a little faster in the expected case, and > allows removing get_user_pages_locked() entirely. Maybe split the lookup_node changes into a separate patch, as that allows to document that change even better.
On 2/3/22 06:01, Christoph Hellwig wrote: > On Thu, Feb 03, 2022 at 01:32:32AM -0800, John Hubbard wrote: >> There is only one caller of get_user_pages_locked(). The purpose of >> get_user_pages_locked() is to allow for unlocking the mmap_lock when >> reading a page from the disk during a page fault (hidden behind >> VM_FAULT_RETRY). The idea is to reduce contention on the heavily-used >> mmap_lock. (Thanks to Jan Kara for clearly pointing that out, and in >> fact I've used some of his wording here.) >> >> However, it is unlikely for lookup_node() to take a page fault. With >> that in mind, change over to calling get_user_pages_fast(). This >> simplifies the code, runs a little faster in the expected case, and >> allows removing get_user_pages_locked() entirely. > > Maybe split the lookup_node changes into a separate patch, as that > allows to document that change even better. OK, I'll do that. thanks,
diff --git a/include/linux/mm.h b/include/linux/mm.h index 80c540c17d83..528ef1cb4f3a 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1916,8 +1916,6 @@ long get_user_pages(unsigned long start, unsigned long nr_pages, long pin_user_pages(unsigned long start, unsigned long nr_pages, unsigned int gup_flags, struct page **pages, struct vm_area_struct **vmas); -long get_user_pages_locked(unsigned long start, unsigned long nr_pages, - unsigned int gup_flags, struct page **pages, int *locked); long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages, struct page **pages, unsigned int gup_flags); long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages, diff --git a/mm/gup.c b/mm/gup.c index b0ecbfe03928..7da49df59110 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2118,65 +2118,6 @@ long get_user_pages(unsigned long start, unsigned long nr_pages, } EXPORT_SYMBOL(get_user_pages); -/** - * get_user_pages_locked() - variant of get_user_pages() - * - * @start: starting user address - * @nr_pages: number of pages from start to pin - * @gup_flags: flags modifying lookup behaviour - * @pages: array that receives pointers to the pages pinned. - * Should be at least nr_pages long. Or NULL, if caller - * only intends to ensure the pages are faulted in. - * @locked: pointer to lock flag indicating whether lock is held and - * subsequently whether VM_FAULT_RETRY functionality can be - * utilised. Lock must initially be held. - * - * It is suitable to replace the form: - * - * mmap_read_lock(mm); - * do_something() - * get_user_pages(mm, ..., pages, NULL); - * mmap_read_unlock(mm); - * - * to: - * - * int locked = 1; - * mmap_read_lock(mm); - * do_something() - * get_user_pages_locked(mm, ..., pages, &locked); - * if (locked) - * mmap_read_unlock(mm); - * - * We can leverage the VM_FAULT_RETRY functionality in the page fault - * paths better by using either get_user_pages_locked() or - * get_user_pages_unlocked(). - * - */ -long get_user_pages_locked(unsigned long start, unsigned long nr_pages, - unsigned int gup_flags, struct page **pages, - int *locked) -{ - /* - * FIXME: Current FOLL_LONGTERM behavior is incompatible with - * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on - * vmas. As there are no users of this flag in this call we simply - * disallow this option for now. - */ - if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM)) - return -EINVAL; - /* - * FOLL_PIN must only be set internally by the pin_user_pages*() APIs, - * never directly by the caller, so enforce that: - */ - if (WARN_ON_ONCE(gup_flags & FOLL_PIN)) - return -EINVAL; - - return __get_user_pages_locked(current->mm, start, nr_pages, - pages, NULL, locked, - gup_flags | FOLL_TOUCH); -} -EXPORT_SYMBOL(get_user_pages_locked); - /* * get_user_pages_unlocked() is suitable to replace the form: * diff --git a/mm/mempolicy.c b/mm/mempolicy.c index 028e8dd82b44..3f8dc58da3e8 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -907,17 +907,14 @@ static void get_policy_nodemask(struct mempolicy *p, nodemask_t *nodes) static int lookup_node(struct mm_struct *mm, unsigned long addr) { struct page *p = NULL; - int err; + int ret; - int locked = 1; - err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked); - if (err > 0) { - err = page_to_nid(p); + ret = get_user_pages_fast(addr & PAGE_MASK, 1, 0, &p); + if (ret > 0) { + ret = page_to_nid(p); put_page(p); } - if (locked) - mmap_read_unlock(mm); - return err; + return ret; } /* Retrieve NUMA policy */ @@ -968,14 +965,14 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask, if (flags & MPOL_F_NODE) { if (flags & MPOL_F_ADDR) { /* - * Take a refcount on the mpol, lookup_node() - * will drop the mmap_lock, so after calling - * lookup_node() only "pol" remains valid, "vma" - * is stale. + * Take a refcount on the mpol, because we are about to + * drop the mmap_lock, after which only "pol" remains + * valid, "vma" is stale. */ pol_refcount = pol; vma = NULL; mpol_get(pol); + mmap_read_unlock(mm); err = lookup_node(mm, addr); if (err < 0) goto out;