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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)



  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

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