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.com>
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
Subject: [PATCH 1/6] MM: improve documentation for __GFP_NOFAIL
Date: Tue, 14 Sep 2021 10:13:04 +1000 [thread overview]
Message-ID: <163157838436.13293.8832201267053160346.stgit@noble.brown> (raw)
In-Reply-To: <163157808321.13293.486682642188075090.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 cost potentially imposed on other subsystems
- 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 | 9 ++++++++-
include/linux/gfp.h | 4 ++++
2 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
index 5954ddf6ee13..9458ce72d31c 100644
--- a/Documentation/core-api/memory-allocation.rst
+++ b/Documentation/core-api/memory-allocation.rst
@@ -126,7 +126,14 @@ 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.
+ The allocator may provide access to memory that would otherwise be
+ reserved in order to satisfy this allocation which might adversely
+ affect other subsystems. So it should only be used when there is no
+ reasonable failure policy and when the memory is likely to be freed
+ again in the near future. Its use is strong discourage (via a
+ WARN_ON) for allocations larger than ``PAGE_ALLOC_COSTLY_ORDER``.
+ While this flag is best avoided, it is still preferable to endless
+ loops around the allocator.
Selecting memory allocator
==========================
diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 55b2ec1f965a..101479373738 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -209,6 +209,10 @@ 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.
+ * Use of this flag may provide access to memory which would otherwise be
+ * reserved. As such it must be understood that there can be a cost imposed
+ * on other subsystems as well as the obvious cost of placing the calling
+ * thread in an uninterruptible indefinite wait.
* Using this flag for costly allocations is _highly_ discouraged.
*/
#define __GFP_IO ((__force gfp_t)___GFP_IO)
next prev parent reply other threads:[~2021-09-14 0:27 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-14 0:13 [PATCH 0/6] congestion_wait() and GFP_NOFAIL NeilBrown
2021-09-14 0:13 ` [PATCH 2/6] MM: annotate congestion_wait() and wait_iff_congested() as ineffective NeilBrown
2021-09-15 11:56 ` Michal Hocko
2021-09-16 22:13 ` NeilBrown
2021-09-14 0:13 ` [PATCH 5/6] XFS: remove congestion_wait() loop from kmem_alloc() NeilBrown
2021-09-14 1:31 ` Dave Chinner
2021-09-14 3:27 ` NeilBrown
2021-09-14 6:05 ` Dave Chinner
2021-09-14 0:13 ` [PATCH 3/6] EXT4: Remove ENOMEM/congestion_wait() loops NeilBrown
2021-09-14 16:34 ` Mel Gorman
2021-09-14 21:48 ` NeilBrown
2021-09-15 12:06 ` Michal Hocko
2021-09-15 22:35 ` NeilBrown
2021-09-16 0:37 ` Dave Chinner
2021-09-16 6:52 ` Michal Hocko
2021-09-14 23:55 ` Dave Chinner
2021-09-15 8:59 ` Mel Gorman
2021-09-15 12:20 ` Michal Hocko
2021-09-15 14:35 ` Mel Gorman
2021-09-15 22:38 ` Dave Chinner
2021-09-16 9:00 ` Mel Gorman
2021-09-15 0:28 ` Theodore Ts'o
2021-09-15 5:25 ` NeilBrown
2021-09-15 17:02 ` Theodore Ts'o
2021-09-14 0:13 ` NeilBrown [this message]
2021-09-15 11:51 ` [PATCH 1/6] MM: improve documentation for __GFP_NOFAIL Michal Hocko
2021-09-14 0:13 ` [PATCH 6/6] XFS: remove congestion_wait() loop from xfs_buf_alloc_pages() NeilBrown
2021-09-14 2:08 ` Dave Chinner
2021-09-14 2:35 ` NeilBrown
2021-09-14 5:33 ` Dave Chinner
2021-09-14 16:45 ` Mel Gorman
2021-09-14 21:13 ` NeilBrown
2021-09-14 0:13 ` [PATCH 4/6] EXT4: remove congestion_wait from ext4_bio_write_page, and simplify NeilBrown
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