linux-kernel - Re: [f2fs-dev] [PATCH v3] f2fs: fix race of pending

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Date:   Wed, 9 Dec 2020 09:34:06 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...nel.org>
CC:     Daeho Jeong <daehojeong@...gle.com>, <kernel-team@...roid.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] [PATCH v3] f2fs: fix race of pending_pages in
 decompression

On 2020/12/9 7:55, Jaegeuk Kim wrote:
> On 12/07, Eric Biggers wrote:
>> On Tue, Dec 08, 2020 at 08:51:45AM +0900, Daeho Jeong wrote:
>>>> I am trying to review this but it is very hard, as the f2fs compression code is
>>>> very hard to understand.
>>>>
>>>> It looks like a 'struct decompress_io_ctx' represents the work to decompress a
>>>> particular cluster.  Since the compressed data of the cluster can be read using
>>>> multiple bios, there is a reference count of how many pages are remaining to be
>>>> read before all the cluster's pages have been read and decompression can start.
>>>>
>>>> What I don't understand is why that reference counting needs to work differently
>>>> depending on whether verity is enabled or not.  Shouldn't it be exactly the
>>>> same?
>>>>
>>>> There also seems to be some confusion about the scope of STEP_VERITY.  Before
>>>> f2fs compression was added, it was a per-bio thing.  But now in a compressed
>>>> file, it's really a per-cluster thing, since all decompressed pages in a
>>>> compressed cluster are verified (or not verified) at once.
>>>>
>>>> Wouldn't it make a lot more sense to, when a cluster needs both compression and
>>>> verity, *not* set STEP_VERITY on the bios, but rather set a similar flag in the
>>>> decompress_io_ctx?
>>>>
>>>
>>> Eric,
>>>
>>> Decompression and verity can be executed in different thread contexts
>>> in different timing, so we need separate counts for each.
>>>
>>> We already use STEP_VERITY for non-compression case, so I think using
>>> this flag in here looks more making sense.
>>>
>>> Thanks,
>>
>> That didn't really answer my questions.
>>
>> I gave up trying to review this patch as the compression post-read handling is
>> just way too weird and hard to understand.  I wrote a patch to clean it all up
>> instead, please take a look:
>> https://lkml.kernel.org/r/20201208060328.2237091-1-ebiggers@kernel.org
> 
> Eric,
> I also tried to review your patch, but it's quite hard to follow quickly and

Me too, it needs more time to check whether the cleanup doesn't miss any cases.

Thanks,

> requires stress tests for a while. Given upcoming merge window and urgency of
> the bug, let me apply Daeho's fix first. By any chance, may I ask revisiting
> your clean-up on top of the fix in the next cycle?
> 
> Thanks,
> 
>>
>> - Eric
> 
> 
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