linux-kernel - Re: [PATCH] MM: discard __GFP

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Date:   Fri, 19 Nov 2021 14:10:18 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     NeilBrown <neilb@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Thierry Reding <thierry.reding@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MM: discard __GFP_ATOMIC

On Fri, Nov 19, 2021 at 10:14:38AM +1100, NeilBrown wrote:
> On Thu, 18 Nov 2021, Matthew Wilcox wrote:
> > Surely this should be gfpflags_allow_blocking() instead of poking about
> > in the innards of gfp flags?
> 
> Possibly.  Didn't know about gfpflags_allow_blocking().  From a quick
> grep in the kernel, a whole lot of other people don't know about it
> either, though clearly some do.
> 
> Maybe we should reaname "__GFP_DIRECT_RECLAIM" to
> "__GFP_ALLOW_BLOCKING", because that is what most users seems to care
> about.

I tend towards the school of thought that the __GFP flags should make
sense to the implementation and users should use either GFP_ or functions.
When we see users adding or subtracting __GFP flags, that's a problem.

> If not, then we probably want a gfpflags_without_block() function that
> removes that flag, as lots of code wants to do that - and using the flag
> for one, and an inline for the other is not consistent.

It's not a _lot_ of code ...

block/bio.c
drivers/net/ethernet/mellanox/mlx4/icm.c
drivers/vhost/net.c
fs/btrfs/disk-io.c
fs/btrfs/volumes.c
fs/erofs/zdata.c
fs/fscache/cookie.c
fs/fscache/page.c
kernel/cgroup/cgroup.c
net/core/skbuff.c
net/core/sock.c
net/netlink/af_netlink.c

(excluding mm -- if mm wants to meddle with GFP flags, that's fine)

I think a lot of these are probably misguided, eg the filesystems should
probably be using GFP_NOFS to prevent recursion.