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: <18532bd8-08bd-4494-a3af-fe252a803380@samsung.com>
Date: Thu, 26 Sep 2024 14:58:31 +0200
From: Daniel Gomez <da.gomez@...sung.com>
To: Matthew Wilcox <willy@...radead.org>, 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>, <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 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?

> 
> 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.
> 

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?

Daniel