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mm: Introduce mm_struct.has_pinned

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(Commit message majorly collected from Jason Gunthorpe)

Reduce the chance of false positive from page_maybe_dma_pinned() by
keeping track if the mm_struct has ever been used with pin_user_pages().
This allows cases that might drive up the page ref_count to avoid any
penalty from handling dma_pinned pages.

Future work is planned, to provide a more sophisticated solution, likely
to turn it into a real counter.  For now, make it atomic_t but use it as
a boolean for simplicity.

Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Peter Xu 2020-09-25 18:25:57 -04:00 committed by Linus Torvalds
parent a1bffa4874
commit 008cfe4418
3 changed files with 17 additions and 0 deletions
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10
include/linux/mm_types.h
View File

@ -436,6 +436,16 @@ struct mm_struct {
*/
atomic_t mm_count;
/**
* @has_pinned: Whether this mm has pinned any pages. This can
* be either replaced in the future by @pinned_vm when it
* becomes stable, or grow into a counter on its own. We're
* aggresive on this bit now - even if the pinned pages were
* unpinned later on, we'll still keep this bit set for the
* lifecycle of this mm just for simplicity.
*/
atomic_t has_pinned;
#ifdef CONFIG_MMU
atomic_long_t pgtables_bytes; /* PTE page table pages */
#endif

1
kernel/fork.c
View File

@ -1011,6 +1011,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mm_pgtables_bytes_init(mm);
mm->map_count = 0;
mm->locked_vm = 0;
atomic_set(&mm->has_pinned, 0);
atomic64_set(&mm->pinned_vm, 0);
memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
spin_lock_init(&mm->page_table_lock);

6
mm/gup.c
View File

@ -1255,6 +1255,9 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
BUG_ON(*locked != 1);
}
if (flags & FOLL_PIN)
atomic_set(&current->mm->has_pinned, 1);
/*
* FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
* is to set FOLL_GET if the caller wants pages[] filled in (but has
@ -2660,6 +2663,9 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
FOLL_FAST_ONLY)))
return -EINVAL;
if (gup_flags & FOLL_PIN)
atomic_set(&current->mm->has_pinned, 1);
if (!(gup_flags & FOLL_FAST_ONLY))
might_lock_read(&current->mm->mmap_lock);