tree e87e8f314985a1ddeb263b478f8455e89a6aa245
parent 6f54cf16fe8c848f0550433f599e4677311574e5
author Linus Torvalds <torvalds@linux-foundation.org> 1555002899 -0700
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> 1557503529 +0200

mm: add 'try_get_page()' helper function

[ Upstream commit 88b1a17dfc3ed7728316478fae0f5ad508f50397 ]

This is the same as the traditional 'get_page()' function, but instead
of unconditionally incrementing the reference count of the page, it only
does so if the count was "safe".  It returns whether the reference count
was incremented (and is marked __must_check, since the caller obviously
has to be aware of it).

Also like 'get_page()', you can't use this function unless you already
had a reference to the page.  The intent is that you can use this
exactly like get_page(), but in situations where you want to limit the
maximum reference count.

The code currently does an unconditional WARN_ON_ONCE() if we ever hit
the reference count issues (either zero or negative), as a notification
that the conditional non-increment actually happened.

NOTE! The count access for the "safety" check is inherently racy, but
that doesn't matter since the buffer we use is basically half the range
of the reference count (ie we look at the sign of the count).

Acked-by: Matthew Wilcox <willy@infradead.org>
Cc: Jann Horn <jannh@google.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>