linux-kernel - Re: [RFC PATCH v2 1/2] mm: shmem: add large folio support to the write and fallocate paths

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Message-ID: <ZvVQoY8Tn_BNc79T@casper.infradead.org>
Date: Thu, 26 Sep 2024 13:16:33 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
	wangkefeng.wang@...wei.com, 21cnbao@...il.com, ryan.roberts@....com,
	ioworker0@...il.com, da.gomez@...sung.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 1/2] mm: shmem: add large folio support to the
 write and fallocate paths

On Thu, Sep 26, 2024 at 04:27:26PM +0800, Baolin Wang wrote:
> +static inline unsigned int
> +shmem_mapping_size_order(struct address_space *mapping, pgoff_t index, size_t size)
> +{
> +	unsigned int order = get_order(max_t(size_t, size, PAGE_SIZE));

Why introduce the max_t() call here?  Did nobody read the documentation
or implementation for get_order() before writing this patch?

Besides, get_order() is wrong (at least relative to other filesystems).
get_order() rounds up instead of down, so what should we do for a write()
of size 512 * 1024 + 1 byte?  Other filesystems allocate an order-8 folio
plus an order-0 folio.  This code would have us allocate an order-9 folio.
I think that's a bad idea.