Re: revisiting alloc_pages_bulks semantics?

[Christoph Hellwig <hch@lst.de>]

On Wed, May 27, 2026 at 04:31:24PM +0800, Zi Yan wrote:
> > Yes, which is really odd, as other page/folio allocators make that an
> > opt-in through GFP flags.
> 
> Based on my understanding of the code, the GFP flags are respected at
> the __alloc_pages_noprof() in alloc_pages_bulk().

As __alloc_pages_noprof is the core of the regular page/folio allocator
I'd expect that as well.

> The loop of
> rmqueue_pcplist() is just a quick try of getting free pages.
> And I suspect it might be quicker than calling __alloc_pages_noprof()
> in a loop, since other preparation work in __alloc_pages_noprof()
> is only done once.

Possibly.  But that means a whole bunch of callers have the wrong
assumption.

> > Well, I really want them.  In some cases I might be fine falling down
> > to smaller sizes, but I also really don't want the logic in every
> > caller.
> 
> Based on your answers above, it sounds like a wrapper of
> __alloc_pages_bulk() that doing allocation in a loop until all requested
> pages are filled might be good enough for your case.
> 
> But let me know if I miss something.

Or just allocate all pages using a loop when alloc_pages_bulk_noprof
doesn't get enough pages from the PCP list?

> > The allocations I have in mind would only require try hard allocations
> > for typical file system blocks sizes (64k at most), while eveything
> > larger is fair game for falling back.
> 
> Sure. In MM, PAGE_ALLOC_COSTLY_ORDER is 3, so pages bigger than that
> would take more effort to get and the allocation latency can be longer.
> So it might take a long time to allocate the last 64KB page in
> a bulk allocation.

Based on the LSF/MM session on lage folios and MM fragmentation session
it seems like we should raise it to 4 for 4k page size platforms,
as this seems to be a proble for 64k folio allocations.