From: Michal Hocko <mhocko@suse.com>
To: NeilBrown <neilb@suse.de>
Cc: 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>,
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: Re: [PATCH 1/6] MM: improve documentation for __GFP_NOFAIL
Date: Wed, 15 Sep 2021 13:51:54 +0200 [thread overview]
Message-ID: <YUHeWnKtR0WMNGo8@dhcp22.suse.cz> (raw)
In-Reply-To: <163157838436.13293.8832201267053160346.stgit@noble.brown>
On Tue 14-09-21 10:13:04, Neil Brown wrote:
> __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.
>
Yes this is a useful addition to the documentation. Thanks!
> Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
> ---
> 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)
>
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-09-15 11:52 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 ` [PATCH 1/6] MM: improve documentation for __GFP_NOFAIL NeilBrown
2021-09-15 11:51 ` Michal Hocko [this message]
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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