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madvise:madvise_free_pte_range(): don't use mapcount() against large folio for sharing check 

commit 0e0e9bd5f7 upstream.

Commit 98b211d641 ("madvise: convert madvise_free_pte_range() to use a
folio") replaced the page_mapcount() with folio_mapcount() to check
whether the folio is shared by other mapping.

It's not correct for large folios. folio_mapcount() returns the total
mapcount of large folio which is not suitable to detect whether the folio
is shared.

Use folio_estimated_sharers() which returns a estimated number of shares.
That means it's not 100% correct. It should be OK for madvise case here.

User-visible effects is that the THP is skipped when user call madvise.
But the correct behavior is THP should be split and processed then.

NOTE: this change is a temporary fix to reduce the user-visible effects
before the long term fix from David is ready.

Link: https://lkml.kernel.org/r/20230808020917.2230692-4-fengwei.yin@intel.com
Fixes: 98b211d641 ("madvise: convert madvise_free_pte_range() to use a folio")
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
Reviewed-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Yin Fengwei 2023-08-08 10:09:17 +08:00 committed by Greg Kroah-Hartman
parent f67e3a725b
commit bd20e20c4d
2 changed files with 21 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
include/linux/mm.h
View File

@ -1727,6 +1727,25 @@ static inline size_t folio_size(struct folio *folio)
return PAGE_SIZE << folio_order(folio);
}
/**
* folio_estimated_sharers - Estimate the number of sharers of a folio.
* @folio: The folio.
*
* folio_estimated_sharers() aims to serve as a function to efficiently
* estimate the number of processes sharing a folio. This is done by
* looking at the precise mapcount of the first subpage in the folio, and
* assuming the other subpages are the same. This may not be true for large
* folios. If you want exact mapcounts for exact calculations, look at
* page_mapcount() or folio_total_mapcount().
*
* Return: The estimated number of processes sharing a folio.
*/
static inline int folio_estimated_sharers(struct folio *folio)
{
return page_mapcount(folio_page(folio, 0));
}
#ifndef HAVE_ARCH_MAKE_PAGE_ACCESSIBLE
static inline int arch_make_page_accessible(struct page *page)
{

4
mm/madvise.c
View File

@ -654,8 +654,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
* deactivate all pages.
*/
if (folio_test_large(folio)) {
if (folio_mapcount(folio) != 1)
goto out;
if (folio_estimated_sharers(folio) != 1)
break;
folio_get(folio);
if (!folio_trylock(folio)) {
folio_put(folio);