Re: [PATCH v4 3/3] mm: warn about illegal __GFP_NOFAIL usage in a more appropriate location and manner

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

On Mon, Sep 2, 2024 at 7:58 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Sat 31-08-24 08:28:23, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > Three points for this change:
> >
> > 1. We should consolidate all warnings in one place. Currently, the
> >    order > 1 warning is in the hotpath, while others are in less
> >    likely scenarios. Moving all warnings to the slowpath will reduce
> >    the overhead for order > 1 and increase the visibility of other
> >    warnings.
> >
> > 2. We currently have two warnings for order: one for order > 1 in
> >    the hotpath and another for order > costly_order in the laziest
> >    path. I suggest standardizing on order > 1 since it’s been in
> >    use for a long time.
> >
> > 3. We don't need to check for __GFP_NOWARN in this case. __GFP_NOWARN
> >    is meant to suppress allocation failure reports, but here we're
> >    dealing with bug detection, not allocation failures. So replace
> >    WARN_ON_ONCE_GFP by WARN_ON_ONCE.
> >
> > Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>
> Acked-by: Michal Hocko <mhocko@suse.com>
>
> Updating the doc about order > 1 sounds like it would still fall into
> the scope of this patch. I don not think we absolutely have to document
> each unsupported gfp flags combination for GFP_NOFAIL but the order is a
> good addition with a note that kvmalloc should be used instead in such a
> case.

Hi Andrew,
If there are no objections from Michal and David, could you please
squash the following:

From fc7a2a49e8d0811d706d13d2080393274f316806 Mon Sep 17 00:00:00 2001
From: Barry Song <v-songbaohua@oppo.com>
Date: Wed, 4 Sep 2024 10:26:19 +1200
Subject: [PATCH] mm: also update the doc for __GFP_NOFAIL with order > 1

Obviously we only support order <= 1 __GFP_NOFAIL allocation and if
someone wants larger memory, they should consider using kvmalloc()
instead.

Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 include/linux/gfp_types.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
index 4a1fa7706b0c..65db9349f905 100644
--- a/include/linux/gfp_types.h
+++ b/include/linux/gfp_types.h
@@ -253,7 +253,8 @@ enum {
  * used only when there is no reasonable failure policy) but it is
  * definitely preferable to use the flag rather than opencode endless
  * loop around allocator.
- * Using this flag for costly allocations is _highly_ discouraged.
+ * Allocating pages from the buddy with __GFP_NOFAIL and order > 1 is
+ * not supported. Please consider using kvmalloc() instead.
  */
 #define __GFP_IO	((__force gfp_t)___GFP_IO)
 #define __GFP_FS	((__force gfp_t)___GFP_FS)
-- 
2.34.1


>
> > ---
> >  mm/page_alloc.c | 50 ++++++++++++++++++++++++-------------------------
> >  1 file changed, 25 insertions(+), 25 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index c81ee5662cc7..e790b4227322 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3033,12 +3033,6 @@ struct page *rmqueue(struct zone *preferred_zone,
> >  {
> >       struct page *page;
> > 
> > -     /*
> > -      * We most definitely don't want callers attempting to
> > -      * allocate greater than order-1 page units with __GFP_NOFAIL.
> > -      */
> > -     WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
> > -
> >       if (likely(pcp_allowed_order(order))) {
> >               page = rmqueue_pcplist(preferred_zone, zone, order,
> >                                      migratetype, alloc_flags);
> > @@ -4175,6 +4169,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >  {
> >       bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
> >       bool can_compact = gfp_compaction_allowed(gfp_mask);
> > +     bool nofail = gfp_mask & __GFP_NOFAIL;
> >       const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
> >       struct page *page = NULL;
> >       unsigned int alloc_flags;
> > @@ -4187,6 +4182,25 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >       unsigned int zonelist_iter_cookie;
> >       int reserve_flags;
> > 
> > +     if (unlikely(nofail)) {
> > +             /*
> > +              * We most definitely don't want callers attempting to
> > +              * allocate greater than order-1 page units with __GFP_NOFAIL.
> > +              */
> > +             WARN_ON_ONCE(order > 1);
> > +             /*
> > +              * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,
> > +              * otherwise, we may result in lockup.
> > +              */
> > +             WARN_ON_ONCE(!can_direct_reclaim);
> > +             /*
> > +              * PF_MEMALLOC request from this context is rather bizarre
> > +              * because we cannot reclaim anything and only can loop waiting
> > +              * for somebody to do a work for us.
> > +              */
> > +             WARN_ON_ONCE(current->flags & PF_MEMALLOC);
> > +     }
> > +
> >  restart:
> >       compaction_retries = 0;
> >       no_progress_loops = 0;
> > @@ -4404,29 +4418,15 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >        * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
> >        * we always retry
> >        */
> > -     if (gfp_mask & __GFP_NOFAIL) {
> > +     if (unlikely(nofail)) {
> >               /*
> > -              * All existing users of the __GFP_NOFAIL are blockable, so warn
> > -              * of any new users that actually require GFP_NOWAIT
> > +              * Lacking direct_reclaim we can't do anything to reclaim memory,
> > +              * we disregard these unreasonable nofail requests and still
> > +              * return NULL
> >                */
> > -             if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
> > +             if (!can_direct_reclaim)
> >                       goto fail;
> > 
> > -             /*
> > -              * PF_MEMALLOC request from this context is rather bizarre
> > -              * because we cannot reclaim anything and only can loop waiting
> > -              * for somebody to do a work for us
> > -              */
> > -             WARN_ON_ONCE_GFP(current->flags & PF_MEMALLOC, gfp_mask);
> > -
> > -             /*
> > -              * non failing costly orders are a hard requirement which we
> > -              * are not prepared for much so let's warn about these users
> > -              * so that we can identify them and convert them to something
> > -              * else.
> > -              */
> > -             WARN_ON_ONCE_GFP(costly_order, gfp_mask);
> > -
> >               /*
> >                * Help non-failing allocations by giving some access to memory
> >                * reserves normally used for high priority non-blocking
> > --
> > 2.34.1
>
> --
> Michal Hocko
> SUSE Labs

Thanks
Barry