Commit c36c04c2 authored by John Hubbard's avatar John Hubbard Committed by Linus Torvalds
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Revert "mm/gup: small refactoring: simplify try_grab_page()" 

This reverts commit 54d516b1

That commit did a refactoring that effectively combined fast and slow
gup paths (again).  And that was again incorrect, for two reasons:

 a) Fast gup and slow gup get reference counts on pages in different
    ways and with different goals: see Linus' writeup in commit
    cd1adf1b ("Revert "mm/gup: remove try_get_page(), call
    try_get_compound_head() directly""), and

 b) try_grab_compound_head() also has a specific check for
    "FOLL_LONGTERM && !is_pinned(page)", that assumes that the caller
    can fall back to slow gup. This resulted in new failures, as
    recently report by Will McVicker [1].

But (a) has problems too, even though they may not have been reported
yet.  So just revert this.

Link: https://lore.kernel.org/r/20220131203504.3458775-1-willmcvicker@google.com

 [1]
Fixes: 54d516b1 ("mm/gup: small refactoring: simplify try_grab_page()")
Reported-and-tested-by: default avatarWill McVicker <willmcvicker@google.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Minchan Kim <minchan@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: stable@vger.kernel.org # 5.15
Signed-off-by: default avatarJohn Hubbard <jhubbard@nvidia.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent d394bb77

mm/gup.c

+30 −5
Original line number Diff line number Diff line
@@ -124,7 +124,7 @@ static inline struct page *try_get_compound_head(struct page *page, int refs) 
 * considered failure, and furthermore, a likely bug in the caller, so a warning

 * is also emitted.

 */

struct page *try_grab_compound_head(struct page *page,

__maybe_unused struct page *try_grab_compound_head(struct page *page,

						   int refs, unsigned int flags)

{

	if (flags & FOLL_GET)
@@ -208,10 +208,35 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags) 
 */

bool __must_check try_grab_page(struct page *page, unsigned int flags)

{

	if (!(flags & (FOLL_GET | FOLL_PIN)))

		return true;

	WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == (FOLL_GET | FOLL_PIN));



	if (flags & FOLL_GET)

		return try_get_page(page);

	else if (flags & FOLL_PIN) {

		int refs = 1;



		page = compound_head(page);



		if (WARN_ON_ONCE(page_ref_count(page) <= 0))

			return false;



		if (hpage_pincount_available(page))

			hpage_pincount_add(page, 1);

		else

			refs = GUP_PIN_COUNTING_BIAS;



		/*

		 * Similar to try_grab_compound_head(): even if using the

		 * hpage_pincount_add/_sub() routines, be sure to

		 * *also* increment the normal page refcount field at least

		 * once, so that the page really is pinned.

		 */

		page_ref_add(page, refs);



		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);

	}



	return try_grab_compound_head(page, 1, flags);

	return true;

}



/**