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Date: Wed, 27 May 2026 14:15:08 +0200
From: Christoph Hellwig <hch@lst.de>
To: Zi Yan <ziy@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Michal Hocko <mhocko@suse.com>,
	Brendan Jackman <jackmanb@google.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	linux-nfs@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: revisiting alloc_pages_bulks semantics?
Message-ID: <20260527121508.GA6079@lst.de>
References: <20260527071816.GA17632@lst.de> <A68C1B33-053C-4406-B78E-871ECD7293B3@nvidia.com> <20260527080056.GA20040@lst.de> <2759BB06-005F-41EF-815F-C9F96E822DE1@nvidia.com>
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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.