This reverts commit 54d516b1d62ff8f17cee2da06e5e4706a0d00b8a

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 cd1adf1b63a1 ("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.

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

Fixes: 54d516b1d62f ("mm/gup: small refactoring: simplify try_grab_page()") Cc: Christoph Hellwig hch@lst.de Tested-by: Will McVicker willmcvicker@google.com 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: Linus Torvalds torvalds@linux-foundation.org Cc: stable@vger.kernel.org # 5.15 Signed-off-by: John Hubbard jhubbard@nvidia.com ---

Since v1, I've only changed the commit description:

* Added Will's Tested-by

* Added Cc: stable

mm/gup.c | 35 ++++++++++++++++++++++++++++++----- 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c index f0af462ac1e2..a9d4d724aef7 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -124,8 +124,8 @@ 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, - int refs, unsigned int flags) +__maybe_unused struct page *try_grab_compound_head(struct page *page, + int refs, unsigned int flags) { if (flags & FOLL_GET) return try_get_compound_head(page, refs); @@ -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));

- return try_grab_compound_head(page, 1, flags); + 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 true; }

/**

base-commit: 9f7fb8de5d9bac17b6392a14af40baf555d9129b