linux-kernel - Re: [PATCH] docs: core-api/gfp_mask-from-fs-io: indicate that vmalloc supports GFP_NOFS/GFP

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Message-ID: <ZzxfvpvfNz9uzkJC@tiehlicka>
Date: Tue, 19 Nov 2024 10:51:58 +0100
From: Michal Hocko <mhocko@...e.com>
To: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Cc: Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] docs: core-api/gfp_mask-from-fs-io: indicate that
 vmalloc supports GFP_NOFS/GFP_NOIO

On Tue 19-11-24 17:39:01, Pavel Tikhomirov wrote:
> After the commit 451769ebb7e79 ("mm/vmalloc: alloc GFP_NO{FS,IO} for
> vmalloc") in v5.17 it is now safe to use GFP_NOFS/GFP_NOIO flags
> in [k]vmalloc, let's reflect it in documentation.
> 
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
>  .../core-api/gfp_mask-from-fs-io.rst          | 20 ++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst
> index e7c32a8de126a..858b2fbcb36c7 100644
> --- a/Documentation/core-api/gfp_mask-from-fs-io.rst
> +++ b/Documentation/core-api/gfp_mask-from-fs-io.rst
> @@ -55,14 +55,16 @@ scope.
>  What about __vmalloc(GFP_NOFS)
>  ==============================
>  
> -vmalloc doesn't support GFP_NOFS semantic because there are hardcoded
> -GFP_KERNEL allocations deep inside the allocator which are quite non-trivial
> -to fix up. That means that calling ``vmalloc`` with GFP_NOFS/GFP_NOIO is
> -almost always a bug. The good news is that the NOFS/NOIO semantic can be
> -achieved by the scope API.
> +Since v5.17, and specifically after the commit 451769ebb7e79 ("mm/vmalloc:
> +alloc GFP_NO{FS,IO} for vmalloc"), GFP_NOFS/GFP_NOIO are now supported in
> +``[k]vmalloc`` by implicitly using scope API.
> +
> +In earlier kernels ``vmalloc`` didn't support GFP_NOFS semantic because there
> +were hardcoded GFP_KERNEL allocations deep inside the allocator. That means
> +that calling ``vmalloc`` with GFP_NOFS/GFP_NOIO was almost always a bug.
>  
>  In the ideal world, upper layers should already mark dangerous contexts
> -and so no special care is required and vmalloc should be called without
> -any problems. Sometimes if the context is not really clear or there are
> -layering violations then the recommended way around that is to wrap ``vmalloc``
> -by the scope API with a comment explaining the problem.
> +and so no special care is required and ``vmalloc`` should be called without any
> +problems. Sometimes if the context is not really clear or there are layering
> +violations then the recommended way around that (on pre-v5.17 kernels) is to
> +wrap ``vmalloc`` by the scope API with a comment explaining the problem.
> -- 
> 2.47.0

-- 
Michal Hocko
SUSE Labs