linux-kernel - Re: [PATCH] erofs: fix two loop issues when read page beyond EOF

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Message-ID: <97875049-8df9-e041-61ca-d90723ba6e82@linux.alibaba.com>
Date:   Sat, 8 Jul 2023 17:00:09 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Chunhai Guo <guochunhai@...o.com>, xiang@...nel.org,
        chao@...nel.org
Cc:     huyue2@...lpad.com, jefflexu@...ux.alibaba.com,
        linux-erofs@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] erofs: fix two loop issues when read page beyond EOF

Hi Chunhai,

On 2023/7/8 14:24, Chunhai Guo wrote:
> When z_erofs_read_folio() reads a page with an offset far beyond EOF, two
> issues may occur:
> - z_erofs_pcluster_readmore() may take a long time to loop when the offset
>    is big enough, which is unnecessary.
>      - For example, it will loop 4691368 times and take about 27 seconds
>        with following case.
>          - offset = 19217289215
>          - inode_size = 1442672
> - z_erofs_do_read_page() may loop infinitely due to the inappropriate
>    truncation in the below statement. Since the offset is 64 bits and
> min_t() truncates the result to 32 bits. The solution is to replace
> unsigned int with another 64-bit type, such as erofs_off_t.
>      cur = end - min_t(unsigned int, offset + end - map->m_la, end);
>      - For example:
>          - offset = 0x400160000
>          - end = 0x370
>          - map->m_la = 0x160370
>          - offset + end - map->m_la = 0x400000000
>          - offset + end - map->m_la = 0x00000000 (truncated as unsigned int)

Thanks for the catch!

Could you split these two into two patches?

how about using:
cur = end - min_t(erofs_off_t, offend + end - map->m_la, end)
for this?

since cur and end are all [0, PAGE_SIZE - 1] for now, and
folio_size() later.

>      - Expected result:
>          - cur = 0
>      - Actual result:
>          - cur = 0x370
> 
> Signed-off-by: Chunhai Guo <guochunhai@...o.com>
> ---
>   fs/erofs/zdata.c | 13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index 5f1890e309c6..6abbd4510076 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -972,7 +972,8 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
>   	struct erofs_map_blocks *const map = &fe->map;
>   	const loff_t offset = page_offset(page);
>   	bool tight = true, exclusive;
> -	unsigned int cur, end, spiltted;
> +	erofs_off_t cur, end;
> +	unsigned int spiltted;
>   	int err = 0;
>   
>   	/* register locked file pages as online pages in pack */
> @@ -1035,7 +1036,7 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
>   	 */
>   	tight &= (fe->mode > Z_EROFS_PCLUSTER_FOLLOWED_NOINPLACE);
>   
> -	cur = end - min_t(unsigned int, offset + end - map->m_la, end);
> +	cur = end - min_t(erofs_off_t, offset + end - map->m_la, end);
>   	if (!(map->m_flags & EROFS_MAP_MAPPED)) {
>   		zero_user_segment(page, cur, end);
>   		goto next_part;
> @@ -1841,7 +1842,7 @@ static void z_erofs_pcluster_readmore(struct z_erofs_decompress_frontend *f,
>   	}
>   
>   	cur = map->m_la + map->m_llen - 1;
> -	while (cur >= end) {
> +	while ((cur >= end) && (cur < i_size_read(inode))) {
>   		pgoff_t index = cur >> PAGE_SHIFT;
>   		struct page *page;
>   
> @@ -1876,6 +1877,12 @@ static int z_erofs_read_folio(struct file *file, struct folio *folio)
>   	trace_erofs_readpage(page, false);
>   	f.headoffset = (erofs_off_t)page->index << PAGE_SHIFT;
>   
> +	/* when trying to read beyond EOF, return zero page directly */
> +	if (f.headoffset >= i_size_read(inode)) {
> +		zero_user_segment(page, 0, PAGE_SIZE);
> +		return 0;
> +	}
Do we really need to optimize this rare case?
I guess the follow readmore fix is enough, thoughts?


Thanks,
Gao Xiang