Re: [PATCH RFC 5/5] non-mm: discourage the usage of __GFP_NOFAIL and encourage GFP_NOFAIL

[Barry Song <21cnbao@gmail.com>]

On Thu, Jul 25, 2024 at 2:41 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Wed, Jul 24, 2024 at 04:39:11PM +0200, Vlastimil Babka wrote:
> > On 7/24/24 3:55 PM, Christoph Hellwig wrote:
> > > On Wed, Jul 24, 2024 at 03:47:46PM +0200, Michal Hocko wrote:
> > >> OK, now it makes more sense ;) I have absolutely no objections to
> > >> prefering scoped NO{FS,IO} interfaces of course. And that would indeed
> > >> eliminate a need for defining GFP_NO{FS,IO}_NOFAIL alternatives.
> > >
> > > Yes.  My proposal would be:
> > >
> > > GFP_NOFAIL without any modifiers it the only valid nofail API.
> >
> > Where GFP_NOFAIL is GFP_KERNEL | __GFP_NOFAIL (and not the more limited one
> > as defined in patch 4/5).
>
> Yes.
>
> > > File systems / drivers can combine іt with the scoped nofs/noio if
> > > needed.
> >
> > Sounds good, how quickly we can convert existing __GFP_NOFAIL users remains
> > to be seen...
>
> I took a quick look at the file system ones and they look pretty easy.  I
> think it would be good to a quick scriped run for everything that does
> GFP_KERNEL | __GFP_NOFAIL right now, and then spend a little time on
> the rest.

I am not quite sure I have understood you, could you please provide a
concrete example,
for example, for the below case?

drivers/md/dm-region-hash.c:            nreg = kmalloc(sizeof(*nreg),
GFP_NOIO | __GFP_NOFAIL);

how are you going to drop the __GFP_IO | __GFP_FS bits while
GFP_NOFAIL = GFP_KERNEL | __GFP_NOFAIL?

And for those cases in which we don't even know GFP_NOIO/GFP_NOFS
is there since gfp is a variable?

gfp |= __GFP_NOFAIL ?