Message-ID: <jk3sbqrkfmtvrzgant74jfm2n3yn6hzd7tefjhjys42yt2trnp@avx5stdnkfsc>
Date: Mon, 14 Jul 2025 11:33:06 +0200
From: Jan Kara <jack@...e.cz>
To: Youling Tang <youling.tang@...ux.dev>
Cc: Matthew Wilcox <willy@...radead.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, chizhiling@....com, 
	Youling Tang <tangyouling@...inos.cn>, Chi Zhiling <chizhiling@...inos.cn>
Subject: Re: [PATCH] mm/filemap: Align last_index to folio size

On Fri 11-07-25 13:55:09, Youling Tang wrote:
> From: Youling Tang <tangyouling@...inos.cn>
> 
> On XFS systems with pagesize=4K, blocksize=16K, and CONFIG_TRANSPARENT_HUGEPAGE
> enabled, We observed the following readahead behaviors:
>  # echo 3 > /proc/sys/vm/drop_caches
>  # dd if=test of=/dev/null bs=64k count=1
>  # ./tools/mm/page-types -r -L -f  /mnt/xfs/test
>  foffset	offset	flags
>  0	136d4c	__RU_l_________H______t_________________F_1
>  1	136d4d	__RU_l__________T_____t_________________F_1
>  2	136d4e	__RU_l__________T_____t_________________F_1
>  3	136d4f	__RU_l__________T_____t_________________F_1
>  ...
>  c	136bb8	__RU_l_________H______t_________________F_1
>  d	136bb9	__RU_l__________T_____t_________________F_1
>  e	136bba	__RU_l__________T_____t_________________F_1
>  f	136bbb	__RU_l__________T_____t_________________F_1   <-- first read
>  10	13c2cc	___U_l_________H______t______________I__F_1   <-- readahead flag
>  11	13c2cd	___U_l__________T_____t______________I__F_1
>  12	13c2ce	___U_l__________T_____t______________I__F_1
>  13	13c2cf	___U_l__________T_____t______________I__F_1
>  ...
>  1c	1405d4	___U_l_________H______t_________________F_1
>  1d	1405d5	___U_l__________T_____t_________________F_1
>  1e	1405d6	___U_l__________T_____t_________________F_1
>  1f	1405d7	___U_l__________T_____t_________________F_1
>  [ra_size = 32, req_count = 16, async_size = 16]
> 
>  # echo 3 > /proc/sys/vm/drop_caches
>  # dd if=test of=/dev/null bs=60k count=1
>  # ./page-types -r -L -f  /mnt/xfs/test
>  foffset	offset	flags
>  0	136048	__RU_l_________H______t_________________F_1
>  ...
>  c	110a40	__RU_l_________H______t_________________F_1
>  d	110a41	__RU_l__________T_____t_________________F_1
>  e	110a42	__RU_l__________T_____t_________________F_1   <-- first read
>  f	110a43	__RU_l__________T_____t_________________F_1   <-- first readahead flag
>  10	13e7a8	___U_l_________H______t_________________F_1
>  ...
>  20	137a00	___U_l_________H______t_______P______I__F_1   <-- second readahead flag (20 - 2f)
>  21	137a01	___U_l__________T_____t_______P______I__F_1
>  ...
>  3f	10d4af	___U_l__________T_____t_______P_________F_1
>  [first readahead: ra_size = 32, req_count = 15, async_size = 17]
> 
> When reading 64k data (same for 61-63k range, where last_index is page-aligned
> in filemap_get_pages()), 128k readahead is triggered via page_cache_sync_ra()
> and the PG_readahead flag is set on the next folio (the one containing 0x10 page).
> 
> When reading 60k data, 128k readahead is also triggered via page_cache_sync_ra().
> However, in this case the readahead flag is set on the 0xf page. Although the
> requested read size (req_count) is 60k, the actual read will be aligned to
> folio size (64k), which triggers the readahead flag and initiates asynchronous
> readahead via page_cache_async_ra(). This results in two readahead operations
> totaling 256k.
> 
> The root cause is that when the requested size is smaller than the actual read
> size (due to folio alignment), it triggers asynchronous readahead. By changing
> last_index alignment from page size to folio size, we ensure the requested size
> matches the actual read size, preventing the case where a single read operation
> triggers two readahead operations.
> 
> After applying the patch:
>  # echo 3 > /proc/sys/vm/drop_caches
>  # dd if=test of=/dev/null bs=60k count=1
>  # ./page-types -r -L -f  /mnt/xfs/test
>  foffset	offset	flags
>  0	136d4c	__RU_l_________H______t_________________F_1
>  1	136d4d	__RU_l__________T_____t_________________F_1
>  2	136d4e	__RU_l__________T_____t_________________F_1
>  3	136d4f	__RU_l__________T_____t_________________F_1
>  ...
>  c	136bb8	__RU_l_________H______t_________________F_1
>  d	136bb9	__RU_l__________T_____t_________________F_1
>  e	136bba	__RU_l__________T_____t_________________F_1   <-- first read
>  f	136bbb	__RU_l__________T_____t_________________F_1
>  10	13c2cc	___U_l_________H______t______________I__F_1   <-- readahead flag
>  11	13c2cd	___U_l__________T_____t______________I__F_1
>  12	13c2ce	___U_l__________T_____t______________I__F_1
>  13	13c2cf	___U_l__________T_____t______________I__F_1
>  ...
>  1c	1405d4	___U_l_________H______t_________________F_1
>  1d	1405d5	___U_l__________T_____t_________________F_1
>  1e	1405d6	___U_l__________T_____t_________________F_1
>  1f	1405d7	___U_l__________T_____t_________________F_1
>  [ra_size = 32, req_count = 16, async_size = 16]
> 
> The same phenomenon will occur when reading from 49k to 64k. Set the readahead
> flag to the next folio.
> 
> Because the minimum order of folio in address_space equals the block size (at
> least in xfs and bcachefs that already support bs > ps), having request_count
> aligned to block size will not cause overread.
> 
> Co-developed-by: Chi Zhiling <chizhiling@...inos.cn>
> Signed-off-by: Chi Zhiling <chizhiling@...inos.cn>
> Signed-off-by: Youling Tang <tangyouling@...inos.cn>

I agree with analysis of the problem but not quite with the solution. See
below.

> diff --git a/mm/filemap.c b/mm/filemap.c
> index 765dc5ef6d5a..56a8656b6f86 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2584,8 +2584,9 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
>  	unsigned int flags;
>  	int err = 0;
>  
> -	/* "last_index" is the index of the page beyond the end of the read */
> -	last_index = DIV_ROUND_UP(iocb->ki_pos + count, PAGE_SIZE);
> +	/* "last_index" is the index of the folio beyond the end of the read */
> +	last_index = round_up(iocb->ki_pos + count, mapping_min_folio_nrbytes(mapping));
> +	last_index >>= PAGE_SHIFT;

I think that filemap_get_pages() shouldn't be really trying to guess what
readahead code needs and round last_index based on min folio order. After
all the situation isn't special for LBS filesystems. It can also happen
that the readahead mark ends up in the middle of large folio for other
reasons. In fact, we already do have code in page_cache_ra_order() ->
ra_alloc_folio() that handles rounding of index where mark should be placed
so your changes essentially try to outsmart that code which is not good. I
think the solution should really be placed in page_cache_ra_order() +
ra_alloc_folio() instead.

In fact the problem you are trying to solve was kind of introduced (or at
least made more visible) by my commit ab4443fe3ca62 ("readahead: avoid
multiple marked readahead pages"). There I've changed the code to round the
index down because I've convinced myself it doesn't matter and rounding
down is easier to handle in that place. But your example shows there are
cases where rounding down has weird consequences and rounding up would have
been better. So I think we need to come up with a method how to round up
the index of marked folio to fix your case without reintroducing problems
mentioned in commit ab4443fe3ca62.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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