From: NeilBrown <neilb@suse.de>
To: Andrew Morton <akpm@linux-foundation.org>,
Theodore Ts'o <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
"Darrick J. Wong" <djwong@kernel.org>,
Matthew Wilcox <willy@infradead.org>,
Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@suse.com>,
". Dave Chinner" <david@fromorbit.com>,
Jonathan Corbet <corbet@lwn.net>
Cc: linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-doc@vger.kernel.org
Subject: [PATCH 2/6] MM: improve documentation for __GFP_NOFAIL
Date: Fri, 17 Sep 2021 12:56:57 +1000 [thread overview]
Message-ID: <163184741778.29351.16920832234899124642.stgit@noble.brown> (raw)
In-Reply-To: <163184698512.29351.4735492251524335974.stgit@noble.brown>
__GFP_NOFAIL is documented both in gfp.h and memory-allocation.rst.
The details are not entirely consistent.
This patch ensures both places state that:
- there is a risk of deadlock with reclaim/writeback/oom-kill
- it should only be used when there is no real alternative
- it is preferable to an endless loop
- it is strongly discourages for costly-order allocations.
Signed-off-by: NeilBrown <neilb@suse.de>
---
Documentation/core-api/memory-allocation.rst | 25 ++++++++++++++++++++++++-
include/linux/gfp.h | 6 +++++-
2 files changed, 29 insertions(+), 2 deletions(-)
diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
index 5954ddf6ee13..8ea077465446 100644
--- a/Documentation/core-api/memory-allocation.rst
+++ b/Documentation/core-api/memory-allocation.rst
@@ -126,7 +126,30 @@ or another request.
* ``GFP_KERNEL | __GFP_NOFAIL`` - overrides the default allocator behavior
and all allocation requests will loop endlessly until they succeed.
- This might be really dangerous especially for larger orders.
+ Any attempt to use ``__GFP_NOFAIL`` for allocations larger than
+ order-1 (2 pages) will trigger a warning.
+
+ Use of ``__GFP_NOFAIL`` can cause deadlocks so it should only be used
+ when there is no alternative, and then should be used with caution.
+ Deadlocks can happen if the calling process holds any resources
+ (e.g. locks) which might be needed for memory reclaim or write-back,
+ or which might prevent a process killed by the OOM killer from
+ successfully exiting. Where possible, locks should be released
+ before using ``__GFP_NOFAIL``.
+
+ While this flag is best avoided, it is still preferable to endless
+ loops around the allocator. Endless loops may still be used when
+ there is a need to test for the process being killed
+ (fatal_signal_pending(current)).
+
+ * ``GFP_NOFS | __GFP_NOFAIL`` - Loop endlessly instead of failing
+ when performing allocations in file system code. The same guidance
+ as for ``GFP_KERNEL | __GFP_NOFAIL`` applies with extra emphasis on
+ the possibility of deadlocks. ``GFP_NOFS`` often implies that
+ filesystem locks are held which might lead to blocking reclaim.
+ Preemptively flushing or reclaiming memory associated with such
+ locks might be appropriate before requesting a ``__GFP_NOFAIL``
+ allocation.
Selecting memory allocator
==========================
diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 55b2ec1f965a..1d2a89e20b8b 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -209,7 +209,11 @@ struct vm_area_struct;
* used only when there is no reasonable failure policy) but it is
* definitely preferable to use the flag rather than opencode endless
* loop around allocator.
- * Using this flag for costly allocations is _highly_ discouraged.
+ * Use of this flag may lead to deadlocks if locks are held which would
+ * be needed for memory reclaim, write-back, or the timely exit of a
+ * process killed by the OOM-killer. Dropping any locks not absolutely
+ * needed is advisable before requesting a %__GFP_NOFAIL allocate.
+ * Using this flag for costly allocations (order>1) is _highly_ discouraged.
*/
#define __GFP_IO ((__force gfp_t)___GFP_IO)
#define __GFP_FS ((__force gfp_t)___GFP_FS)
next prev parent reply other threads:[~2021-09-17 2:59 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-17 2:56 [PATCH 0/6 v2] congestion_wait() and GFP_NOFAIL NeilBrown
2021-09-17 2:56 ` [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages() NeilBrown
2021-09-17 2:56 ` [PATCH 5/6] XFS: remove congestion_wait() loop from kmem_alloc() NeilBrown
2021-09-17 21:45 ` Dave Chinner
2021-09-17 2:56 ` [PATCH 1/6] MM: Support __GFP_NOFAIL in alloc_pages_bulk_*() and improve doco NeilBrown
2021-09-17 14:42 ` Mel Gorman
2021-09-20 23:48 ` NeilBrown
2021-10-05 9:16 ` Vlastimil Babka
2021-09-17 2:56 ` [PATCH 3/6] EXT4: Remove ENOMEM/congestion_wait() loops NeilBrown
2021-09-17 2:56 ` [PATCH 4/6] EXT4: remove congestion_wait from ext4_bio_write_page, and simplify NeilBrown
2021-09-17 2:56 ` NeilBrown [this message]
2021-10-05 9:20 ` [PATCH 2/6] MM: improve documentation for __GFP_NOFAIL Vlastimil Babka
2021-10-05 11:09 ` Michal Hocko
2021-10-05 12:27 ` Vlastimil Babka
2021-10-06 23:14 ` Dave Chinner
2021-10-07 10:07 ` Michal Hocko
2021-10-07 23:15 ` NeilBrown
2021-10-08 7:48 ` Michal Hocko
2021-10-08 22:36 ` Dave Chinner
2021-10-11 11:57 ` Michal Hocko
2021-10-11 21:49 ` NeilBrown
2021-10-18 10:23 ` Michal Hocko
2021-10-19 4:32 ` NeilBrown
2021-10-19 13:59 ` Michal Hocko
2021-10-22 0:09 ` NeilBrown
2021-10-13 2:32 ` Dave Chinner
2021-10-13 8:26 ` Michal Hocko
2021-10-14 11:32 ` David Sterba
2021-10-14 11:46 ` Michal Hocko
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=163184741778.29351.16920832234899124642.stgit@noble.brown \
--to=neilb@suse.de \
--cc=adilger.kernel@dilger.ca \
--cc=akpm@linux-foundation.org \
--cc=corbet@lwn.net \
--cc=david@fromorbit.com \
--cc=djwong@kernel.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-ext4@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-nfs@vger.kernel.org \
--cc=linux-xfs@vger.kernel.org \
--cc=mgorman@suse.de \
--cc=mhocko@suse.com \
--cc=tytso@mit.edu \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).