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

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Message-ID: <20200712082123.GA11082@kernel.org>
Date:   Sun, 12 Jul 2020 11:21:23 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Jonathan Corbet <corbet@....net>
Cc:     Michal Hocko <mhocko@...e.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH] docs/core-api: memory-allocation: describe reclaim
 behaviour

Another ping :)

On Fri, Jul 03, 2020 at 12:55:04AM +0300, Mike Rapoport wrote:
> Gentle ping.
> 
> On Fri, Jun 26, 2020 at 05:29:50PM +0300, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@...ux.ibm.com>
> > 
> > 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@...e.com>
> > Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> > ---
> > 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
> > 
> 
> -- 
> Sincerely yours,
> Mike.

-- 
Sincerely yours,
Mike.