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mm/gup: reorganize internal_get_user_pages_fast() 

Patch series "Add a seqcount between gup_fast and copy_page_range()", v4.

As discussed and suggested by Linus use a seqcount to close the small race
between gup_fast and copy_page_range().

Ahmed confirms that raw_write_seqcount_begin() is the correct API to use
in this case and it doesn't trigger any lockdeps.

I was able to test it using two threads, one forking and the other using
ibv_reg_mr() to trigger GUP fast.  Modifying copy_page_range() to sleep
made the window large enough to reliably hit to test the logic.

This patch (of 2):

The next patch in this series makes the lockless flow a little more
complex, so move the entire block into a new function and remove a level
of indention.  Tidy a bit of cruft:

 - addr is always the same as start, so use start

 - Use the modern check_add_overflow() for computing end = start + len

 - nr_pinned/pages << PAGE_SHIFT needs the LHS to be unsigned long to
   avoid shift overflow, make the variables unsigned long to avoid coding
   casts in both places. nr_pinned was missing its cast

 - The handling of ret and nr_pinned can be streamlined a bit

No functional change.

Link: https://lkml.kernel.org/r/0-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
Link: https://lkml.kernel.org/r/1-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Jason Gunthorpe 2020-12-14 19:05:41 -08:00 committed by Linus Torvalds
parent d0de824118
commit c28b1fc703
1 changed files with 54 additions and 45 deletions
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99
mm/gup.c
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@ -2677,13 +2677,43 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
return ret;
}
static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
static unsigned long lockless_pages_from_mm(unsigned long start,
unsigned long end,
unsigned int gup_flags,
struct page **pages)
{
unsigned long flags;
int nr_pinned = 0;
if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
!gup_fast_permitted(start, end))
return 0;
/*
* Disable interrupts. The nested form is used, in order to allow full,
* general purpose use of this routine.
*
* With interrupts disabled, we block page table pages from being freed
* from under us. See struct mmu_table_batch comments in
* include/asm-generic/tlb.h for more details.
*
* We do not adopt an rcu_read_lock() here as we also want to block IPIs
* that come from THPs splitting.
*/
local_irq_save(flags);
gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
local_irq_restore(flags);
return nr_pinned;
}
static int internal_get_user_pages_fast(unsigned long start,
unsigned long nr_pages,
unsigned int gup_flags,
struct page **pages)
{
unsigned long addr, len, end;
unsigned long flags;
int nr_pinned = 0, ret = 0;
unsigned long len, end;
unsigned long nr_pinned;
int ret;
if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
FOLL_FORCE | FOLL_PIN | FOLL_GET |
@ -2697,54 +2727,33 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
might_lock_read(&current->mm->mmap_lock);
start = untagged_addr(start) & PAGE_MASK;
addr = start;
len = (unsigned long) nr_pages << PAGE_SHIFT;
end = start + len;
if (end <= start)
len = nr_pages << PAGE_SHIFT;
if (check_add_overflow(start, len, &end))
return 0;
if (unlikely(!access_ok((void __user *)start, len)))
return -EFAULT;
/*
* Disable interrupts. The nested form is used, in order to allow
* full, general purpose use of this routine.
*
* With interrupts disabled, we block page table pages from being
* freed from under us. See struct mmu_table_batch comments in
* include/asm-generic/tlb.h for more details.
*
* We do not adopt an rcu_read_lock(.) here as we also want to
* block IPIs that come from THPs splitting.
*/
if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) {
unsigned long fast_flags = gup_flags;
nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
return nr_pinned;
local_irq_save(flags);
gup_pgd_range(addr, end, fast_flags, pages, &nr_pinned);
local_irq_restore(flags);
ret = nr_pinned;
/* Slow path: try to get the remaining pages with get_user_pages */
start += nr_pinned << PAGE_SHIFT;
pages += nr_pinned;
ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
pages);
if (ret < 0) {
/*
* The caller has to unpin the pages we already pinned so
* returning -errno is not an option
*/
if (nr_pinned)
return nr_pinned;
return ret;
}
if (nr_pinned < nr_pages && !(gup_flags & FOLL_FAST_ONLY)) {
/* Try to get the remaining pages with get_user_pages */
start += nr_pinned << PAGE_SHIFT;
pages += nr_pinned;
ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned,
gup_flags, pages);
/* Have to be a bit careful with return values */
if (nr_pinned > 0) {
if (ret < 0)
ret = nr_pinned;
else
ret += nr_pinned;
}
}
return ret;
return ret + nr_pinned;
}
/**
* get_user_pages_fast_only() - pin user pages in memory
* @start: starting user address