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Date: Tue, 21 Mar 2023 09:29:39 +0800 From: Zhihao Cheng <chengzhihao1@...wei.com> To: Jan Kara <jack@...e.cz> CC: <tytso@....edu>, <adilger.kernel@...ger.ca>, <jack@...e.com>, <tudor.ambarus@...aro.org>, <linux-ext4@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com> Subject: Re: [PATCH] ext4: Fix i_disksize exceeding i_size problem in paritally written case > On Mon 20-03-23 20:49:07, Zhihao Cheng wrote: >> [...] >> >>>> BTW, I want send another patch as follows: >>>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c >>>> index bf0b7dea4900..570a687ae847 100644 >>>> --- a/fs/ext4/inode.c >>>> +++ b/fs/ext4/inode.c >>>> @@ -3149,7 +3149,7 @@ static int ext4_da_write_end(struct file *file, >>>> return ext4_write_inline_data_end(inode, pos, len, copied, >>>> page); >>>> >>>> start = pos & (PAGE_SIZE - 1); >>>> - end = start + copied - 1; >>>> + end = start + (copied ? copied - 1 : copied); >>>> >>>> /* >>>> * Since we are holding inode lock, we are sure i_disksize <= >>>> @@ -3167,7 +3167,7 @@ static int ext4_da_write_end(struct file *file, >>>> * ext4_da_write_inline_data_end(). >>>> */ >>>> new_i_size = pos + copied; >>>> - if (copied && new_i_size > inode->i_size && >>>> + if (new_i_size > inode->i_size && >>>> ext4_da_should_update_i_disksize(page, end)) >>>> ext4_update_i_disksize(inode, new_i_size); >>>> >>>> This modification handle unconsistent i_size and i_disksize imported by >>>> ea51d132dbf9 ("ext4: avoid hangs in ext4_da_should_update_i_disksize()"). >>>> >>>> Paritially written may display a fake inode size for user, for example: >>>> >>>> >>>> >>>> i_disksize=1 >>>> >>>> generic_perform_write >>>> >>>> copied = iov_iter_copy_from_user_atomic(len) // copied = 0 >>>> >>>> ext4_da_write_end // skip updating i_disksize >>>> >>>> generic_write_end >>>> >>>> if (pos + copied > inode->i_size) { // 10 + 0 > 1, true >>>> >>>> i_size_write(inode, pos + copied); // i_size = 10 >>>> >>>> } >>>> >>>> >>>> >>>> 0 1 10 4096 >>>> >>>> |_|_______|_________..._____| >>>> >>>> | | >>>> >>>> i_size pos >>>> >>>> >>>> >>>> Now, user see the i_size is 10 (i_disksize is still 1). After inode >>>> >>>> destroyed, user will get the i_size is 1 read from disk. >>> >>> OK, but shouldn't we rather change generic_write_end() to not increase >>> i_size if no write happened? Because that is what seems somewhat >>> problematic... >>> >>> Honza >>> >> >> After looking through some code, I find some other places have similar >> problem: >> 1. In ext4_write_end(), i_size is updated by ext4 not generic_write_end(). >> 2. The iommap framework, i_size is updated even copied is zero. >> 3. ubifs_write_end, i_size is updated even copied is zero. >> >> It seems that fixing all places is not an easy work. > > Well, yeah, probably not trivial but still desirable ;). Will you look into > that? > > Honza > I'am happy to investigate it, maybe it will take some time, and I'm also glad to help review code if somebody come up a solution firstly.
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