Re: [PATCH] docs/core-api: memory-allocation: describe reclaim behaviour

[Michal Hocko]

On Fri 26-06-20 17:29:50, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> 
> Changelog of commit dcda9b04713c ("mm, tree wide: replace __GFP_REPEAT by
> __GFP_RETRY_MAYFAIL with more useful semantic") has very nice description
> of GFP flags that affect reclaim behaviour of the page allocator.
> 
> It would be pity to keep this description buried in the log so let's expose
> it in the Documentation/ as well.
> 
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>

Thanks for making that into the documentation.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> Hi,
> 
> I've been looking for something completely unrealated and found this
> really nice piece of documentation.
> 
> Thanks Michal! ;-)
> 
>  Documentation/core-api/memory-allocation.rst | 44 ++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> index 4aa82ddd01b8..4446a1ac36cc 100644
> --- a/Documentation/core-api/memory-allocation.rst
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -84,6 +84,50 @@ driver for a device with such restrictions, avoid using these flags.
>  And even with hardware with restrictions it is preferable to use
>  `dma_alloc*` APIs.
>  
> +GFP flags and reclaim behavior
> +------------------------------
> +Memory allocations may trigger direct or background reclaim and it is
> +useful to understand how hard the page allocator will try to satisfy that
> +or another request.
> +
> +  * ``GFP_KERNEL & ~__GFP_RECLAIM`` - optimistic allocation without _any_
> +    attempt to free memory at all. The most light weight mode which even
> +    doesn't kick the background reclaim. Should be used carefully because it
> +    might deplete the memory and the next user might hit the more aggressive
> +    reclaim.
> +
> +  * ``GFP_KERNEL & ~__GFP_DIRECT_RECLAIM`` (or ``GFP_NOWAIT``)- optimistic
> +    allocation without any attempt to free memory from the current
> +    context but can wake kswapd to reclaim memory if the zone is below
> +    the low watermark. Can be used from either atomic contexts or when
> +    the request is a performance optimization and there is another
> +    fallback for a slow path.
> +
> +  * ``(GFP_KERNEL|__GFP_HIGH) & ~__GFP_DIRECT_RECLAIM`` (aka ``GFP_ATOMIC``) -
> +    non sleeping allocation with an expensive fallback so it can access
> +    some portion of memory reserves. Usually used from interrupt/bottom-half
> +    context with an expensive slow path fallback.
> +
> +  * ``GFP_KERNEL`` - both background and direct reclaim are allowed and the
> +    **default** page allocator behavior is used. That means that not costly
> +    allocation requests are basically no-fail but there is no guarantee of
> +    that behavior so failures have to be checked properly by callers
> +    (e.g. OOM killer victim is allowed to fail currently).
> +
> +  * ``GFP_KERNEL | __GFP_NORETRY`` - overrides the default allocator behavior
> +    and all allocation requests fail early rather than cause disruptive
> +    reclaim (one round of reclaim in this implementation). The OOM killer
> +    is not invoked.
> +
> +  * ``GFP_KERNEL | __GFP_RETRY_MAYFAIL`` - overrides the default allocator
> +    behavior and all allocation requests try really hard. The request
> +    will fail if the reclaim cannot make any progress. The OOM killer
> +    won't be triggered.
> +
> +  * ``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.
> +
>  Selecting memory allocator
>  ==========================
>  
> -- 
> 2.25.4

-- 
Michal Hocko
SUSE Labs