On Wed 31-07-24 19:08:44, Barry Song wrote:
> On Wed, Jul 31, 2024 at 6:55 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> >
> > On 7/31/24 2:01 AM, Barry Song wrote:
> > > From: Barry Song <v-songbaohua@xxxxxxxx>
> > >
> > > When users allocate memory with the __GFP_NOFAIL flag, they might
> > > incorrectly use it alongside GFP_ATOMIC, GFP_NOWAIT, etc. This kind
> > > of non-blockable __GFP_NOFAIL is not supported and is pointless. If
> > > we attempt and still fail to allocate memory for these users, we have
> > > two choices:
> > >
> > > 1. We could busy-loop and hope that some other direct reclamation or
> > > kswapd rescues the current process. However, this is unreliable
> > > and could ultimately lead to hard or soft lockups, which might not
> > > be well supported by some architectures.
> > >
> > > 2. We could use BUG_ON to trigger a reliable system crash, avoiding
> > > exposing NULL dereference.
> > >
> > > This patch chooses the second option because the first is unreliable. Even
> > > if the process incorrectly using __GFP_NOFAIL is sometimes rescued, the
> > > long latency might be unacceptable, especially considering that misusing
> > > GFP_ATOMIC and __GFP_NOFAIL is likely to occur in atomic contexts with
> > > strict timing requirements.
> > >
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> > > Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > > Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> > > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > > Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> > > Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> > > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Kees Cook <kees@xxxxxxxxxx>
> > > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > > ---
> > > mm/page_alloc.c | 10 +++++-----
> > > 1 file changed, 5 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index cc179c3e68df..ed1bd8f595bd 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -4439,11 +4439,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> > > */
> > > if (gfp_mask & __GFP_NOFAIL) {
> > > /*
> > > - * All existing users of the __GFP_NOFAIL are blockable, so warn
> > > - * of any new users that actually require GFP_NOWAIT
> > > + * All existing users of the __GFP_NOFAIL are blockable
> > > + * otherwise we introduce a busy loop with inside the page
> > > + * allocator from non-sleepable contexts
> > > */
> > > - if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
> > > - goto fail;
> > > + BUG_ON(!can_direct_reclaim);
> >
> > We might get more useful output if here we did just "if
> > (!can_direct_reclaim) goto fail; and let warn_alloc() print it, and then
> > there would be a BUG_ON(gfp_mask & __GFP_NOFAIL)?
> > Additionally we could mask out __GFP_NOWARN from gfp_mask before the goto,
> > as a __GFP_NOWARN would suppress the output in a non-recoverable situation
> > so it would be wrong.
>
> If we use BUG_ON, it seems like we don't need to do anything else, as the BUG_ON
> report gives developers all the information they need.
It will not give warn_alloc - aka state of the page allocator at the
time of failure. Is this really necessary? I don't know because it is
"shouldn't ever happen" rather than "how come this allocation has
failed" case.
So IMHO a simple BUG_ON should be sufficient to scream out loud that
impossible has happened and need fixing.
--
Michal Hocko
SUSE Labs
