On Wed 31-07-24 12:01:55, 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.
Well, any latency arguments are out of table with BUG_ON crashing the
system. So this is not about reliability but rather making those
incorrect uses more obvious.
With your GFP_NOFAIL follow up this should be simply impossible to
trigger though. I am still not sure which of the bad solutions is more
appropriate so I am not giving this an ack. Either of them is better
than allow to fail though.
> 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);
>
> /*
> * PF_MEMALLOC request from this context is rather bizarre
> @@ -4474,7 +4474,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> cond_resched();
> goto retry;
> }
> -fail:
> +
> warn_alloc(gfp_mask, ac->nodemask,
> "page allocation failure: order:%u", order);
> got_pg:
> --
> 2.34.1
--
Michal Hocko
SUSE Labs
