Re: [PATCH v3 19/26] mm/page_alloc: rename ALLOC_NON_BLOCK back to _HARDER

On Fri, Jul 31, 2026 at 04:52:59PM +0200, Vlastimil Babka (SUSE) wrote:
> On 7/27/26 00:22, Brendan Jackman wrote:
> > Commit 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH
> > non-blocking allocations accesses reserves") renamed ALLOC_HARDER to
> > ALLOC_NON_BLOCK because the former is "a vague description".
> > 
> > However, vagueness is accurate here, this is a vague flag. It is not set
> > for __GFP_NOMEMALLOC. It doesn't really mean "allocate without blocking"
> > but rather "allow dipping into atomic reserves, _because_ of the need
> > not to block".
> > 
> > A later commit will need an alloc flag that really means "don't block
> > here", so go back to the flag's old name and update the commentary
> > to try and give it a slightly clearer meaning.
> > 
> > Signed-off-by: Brendan Jackman <[email protected]>
> 
> I wonder if we need to do this, and instead we could repurpose
> ALLOC_NON_BLOCK directly. AFAIU it's about removing the side-effect of
> gfp_allowed_mask in the next patch. But what would happen if we did that
> using the existing ALLOC_NON_BLOCK (or maybe just renamed to ALLOC_NOBLOCK?).
> 
> - in __zone_watermark_ok(), ALLOC_NON_BLOCK could now be *not* set in
> situations where previously it was set due to gfp_allowed_mask masking out
> __GFP_DIRECT_RECLAIM. But it only has an effect on top of __GFP_HIGH (thus
> ALLOC_MIN_RESERVE... which seems contradicting the ALLOC_NON_BLOCK
> description comment btw). Also __GFP_DIRECT_RECLAIM is only masked out by
> GFP_BOOT_MASK when all memory is free, so it's kinda moot?
> 
> - in rmqueue_buddy() we allow access to highatomic reserves since
> 281dd25c1a018. That commit describes GFP_ATOMIC so we could have been
> checking ALLOC_MIN_RESERVE. But we can also leave this alone because it
> doesn't actually matter when GFP_BOOT_MASK is set, as above.

IIUC we are trying to find out if the callers either has interrupts
disabled or is holding a lock, and using __GFP_DIRECT_RECLAIM as an
indicator. As you mention, it seems like __GFP_DIRECT_RECLAIM is only
masked during boot, presumably before we can allocate any unmapped
memory (should always be user memory?).

So maybe we should just use gfpflags_allow_blocking()?

I also wonder if restricing to callers __GFP_DIRECT_RECLAIM is too
restrictive, we can probably key off __GFP_ATOMIC as I assume any
callers with IRQs disabled or wiht a lock have to set it.

Maybe we can also use preemptible(), but that creates a dependency on
CONFIG_PREEMPT_COUNT as far as I can tell.