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@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>
> > ---
> > 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.
