linux-kernel - Re: [PATCH v2 7/7] docs/core-api: mm-api: add section about GFP flags

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Date:   Thu, 26 Jul 2018 08:36:05 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     Michal Hocko <mhocko@...nel.org>, Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/7] docs/core-api: mm-api: add section about GFP flags

On Thu, Jul 26, 2018 at 06:18:53PM +0300, Mike Rapoport wrote:
> On Thu, Jul 26, 2018 at 04:20:39PM +0200, Michal Hocko wrote:
> > >   Functions which need to allocate memory often use GFP flags to express
> > >   how that memory should be allocated.  The GFP acronym stands for "get
> > >   free pages", the underlying memory allocation function.
> > 
> > OK.
> > 
> > >   Not every GFP
> > >   flag is allowed to every function which may allocate memory.  Most
> > >   users will want to use a plain ``GFP_KERNEL`` or ``GFP_ATOMIC``.
> > 
> > Or rather than mentioning the two just use "Useful GFP flag
> > combinations" comment segment from gfp.h
> 
> The comment there includes GFP_DMA, GFP_NOIO etc so I'd prefer Matthew's
> version and maybe even omit GFP_ATOMIC from it.

I'm totally OK with that.

> Some grepping shows that roughly 80% of allocations are GFP_KERNEL, 12% are
> GFP_ATOMIC and ... I didn't count the usage of other flags ;-)

;-)  You'll find a lot of GFP_NOFS and GFP_NOIO in the filesystem/block
code ...