On Tue, Feb 1, 2022 at 10:00 AM Will McVicker willmcvicker@google.com wrote:

On Tue, Feb 1, 2022 at 1:29 AM John Hubbard jhubbard@nvidia.com wrote:

...

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 Cc: 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 Signed-off-by: John Hubbard jhubbard@nvidia.com

---

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: 26291c54e111ff6ba87a164d85d4a4e134b7315c

2.35.1

Thanks John! I verified this works on the Pixel 6 with the 5.15 kernel for my camera use-case. Free free to include:

Tested-by: Will McVicker willmcvicker@google.com

Thanks, Will

And just so we don't miss this, I'd also like to request this be pulled into the 5.15 stable branch please.

Cc: stable@vger.kernel.org # 5.15

Thanks, Will