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mm: speculative page references

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If we can be sure that elevating the page_count on a pagecache page will
pin it, we can speculatively run this operation, and subsequently check to
see if we hit the right page rather than relying on holding a lock or
otherwise pinning a reference to the page.

This can be done if get_page/put_page behaves consistently throughout the
whole tree (ie.  if we "get" the page after it has been used for something
else, we must be able to free it with a put_page).

Actually, there is a period where the count behaves differently: when the
page is free or if it is a constituent page of a compound page.  We need
an atomic_inc_not_zero operation to ensure we don't try to grab the page
in either case.

This patch introduces the core locking protocol to the pagecache (ie.
adds page_cache_get_speculative, and tweaks some update-side code to make
it work).

Thanks to Hugh for pointing out an improvement to the algorithm setting
page_count to zero when we have control of all references, in order to
hold off speculative getters.

[[email protected]: fix migration_entry_wait()]
[[email protected]: fix add_to_page_cache]
[[email protected]: repair a comment]
Signed-off-by: Nick Piggin <[email protected]>
Cc: Jeff Garzik <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Reviewed-by: Peter Zijlstra <[email protected]>
Signed-off-by: Daisuke Nishimura <[email protected]>
Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: KOSAKI Motohiro <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Nick Piggin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Nick Piggin 2008-07-25 19:45:30 -07:00 • committed by Linus Torvalds
commit e286781d5f
7 changed files with 227 additions and 45 deletions
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6
mm/shmem.c
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@ -936,7 +936,7 @@ found:
spin_lock(&info->lock);
ptr = shmem_swp_entry(info, idx, NULL);
if (ptr && ptr->val == entry.val) {
error = add_to_page_cache(page, inode->i_mapping,
error = add_to_page_cache_locked(page, inode->i_mapping,
idx, GFP_NOWAIT);
/* does mem_cgroup_uncharge_cache_page on error */
} else /* we must compensate for our precharge above */
@ -1301,8 +1301,8 @@ repeat:
SetPageUptodate(filepage);
set_page_dirty(filepage);
swap_free(swap);
} else if (!(error = add_to_page_cache(
swappage, mapping, idx, GFP_NOWAIT))) {
} else if (!(error = add_to_page_cache_locked(swappage, mapping,
idx, GFP_NOWAIT))) {
info->flags |= SHMEM_PAGEIN;
shmem_swp_set(info, entry, 0);
shmem_swp_unmap(entry);