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: <ZvVkM0VWK5E_MfJH@casper.infradead.org>
Date: Thu, 26 Sep 2024 14:40:03 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Daniel Gomez <da.gomez@...sung.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
	hughd@...gle.com, david@...hat.com, wangkefeng.wang@...wei.com,
	21cnbao@...il.com, ryan.roberts@....com, ioworker0@...il.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 02:58:31PM +0200, Daniel Gomez wrote:
> On 9/26/2024 2:16 PM, Matthew Wilcox wrote:
> > 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?
> 
> get_order() result is undefined if the size is 0. I've used max_t() here to
> avoid that case. Perhaps should we prevent that case before getting here?

Surely we've handled a length-0 write before we get here?

> I think one of my earlier attemps was to use fgf_set_order + FGF_GET_ORDER()
> as in iomap. But the solution taken there was to share code between shmem
> and filemap and that wasn't considered a good idea. Shall we just replicate
> iomap_get_folio()? Or else, what do you suggest here?

We could move three of the four lines from fgf_set_order() into a
new function and call it from both fgf_set_order() and shmem?
>