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Date: Mon, 25 May 2026 12:35:56 +0300
From: Mike Rapoport <rppt@kernel.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] block: partitions: replace __get_free_page() with
 kmalloc()
Message-ID: <ahQX_JCgS9JWIhY-@kernel.org>
References: <20260520-block-v1-1-6463dc2cf042@kernel.org>
 <ahPnX-CwVgK-9chp@infradead.org>
 <ahPxmSqT4Y4jcAbQ@kernel.org>
 <ahP3R2nH8-bHMVf0@infradead.org>
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In-Reply-To: <ahP3R2nH8-bHMVf0@infradead.org>

On Mon, May 25, 2026 at 12:16:23AM -0700, Christoph Hellwig wrote:
> On Mon, May 25, 2026 at 09:52:09AM +0300, Mike Rapoport wrote:
> > On Sun, May 24, 2026 at 11:08:31PM -0700, Christoph Hellwig wrote:
> > > On Wed, May 20, 2026 at 11:15:52AM +0300, Mike Rapoport (Microsoft) wrote:
> > > > check_partition() allocates a buffer to use as backing buffer for
> > > > seq_buf.
> > > > 
> > > > This buffer can be allocated with kmalloc() as there's nothing special
> > > > about it to go directly to the page allocator.
> > > > 
> > > > Replace use of __get_free_page() with kmalloc() and free_page() with
> > > > kfree().
> > > 
> > > So I heard various vague references that we should replace
> > > __get_free_page with kmalloc, but nothing definitive.  Can you please
> > > point to a good resource for that?
> > 
> > There was quite recent discussion when I posted patches that change
> > __get_free_page to return void *:
> > 
> > https://lore.kernel.org/all/20251018093002.3660549-1-rppt@kernel.org/
> 
> This doesn't tell much more.
> 
> > And an old thread when Al posted similar patches:
> > 
> > https://lore.kernel.org/all/CA+55aFwp4iy4rtX2gE2WjBGFL=NxMVnoFeHqYa2j1dYOMMGqxg@mail.gmail.com/T/#u<S-Del>
> 
> This does, but it still fails to explain why kmalloc performs just as
> well as __get_free_page(s) these days.

I don't think that in this case - a single allocation on the cold path -
the performance difference is even measurable.

Nevertheless allocations from slab caches are way faster than
__get_free_page() (i.e.  alloc_pages()) as it's essentially lockless
cmpxchg. Allocations that need to refill the cache do alloc_pages() with a
little of slab bookkeeping overhead.

-- 
Sincerely yours,
Mike.