linux-kernel - Re: [PATCH v1 4/5] f2fs: add 'folio_in_bio' to handle readahead folios with no BIO submission

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Message-ID: <c83f9368-e962-440c-9107-ba76ca71602a@kernel.org>
Date: Wed, 7 Jan 2026 09:16:50 +0800
From: Chao Yu <chao@...nel.org>
To: Nanzhe Zhao <nzzhao@....com>
Cc: chao@...nel.org, Kim Jaegeuk <jaegeuk@...nel.org>,
 linux-f2fs-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 4/5] f2fs: add 'folio_in_bio' to handle readahead
 folios with no BIO submission

On 1/7/2026 8:33 AM, Nanzhe Zhao wrote:
> Hi Chao yu:
> At 2026-01-06 17:31:20, "Chao Yu" <chao@...nel.org> wrote:
>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>> index 66ab7a43a56f..ac569a396914 100644
>>>> --- a/fs/f2fs/data.c
>>>> +++ b/fs/f2fs/data.c
>>>> @@ -2430,6 +2430,7 @@ static int f2fs_read_data_large_folio(struct inode *inode,
>>>>    	unsigned nrpages;
>>>>    	struct f2fs_folio_state *ffs;
>>>>    	int ret = 0;
>>>> +	bool folio_in_bio = false;
>>>
>>> No need to initialize folio_in_bio?
> 
> Agreed. It's redundant since we reset it to false for each new folio before processing.
> 
>>>> @@ -2539,6 +2542,11 @@ static int f2fs_read_data_large_folio(struct inode *inode,
>>>>    	}
>>>>    	trace_f2fs_read_folio(folio, DATA);
>>>>    	if (rac) {
>>>> +		if (!folio_in_bio) {
>>>> +			if (!ret)
>>>> +				folio_mark_uptodate(folio);
>>>> +			folio_unlock(folio);
>>>> +	}
>>>
>>> err_out:
>>> 	/* Nothing was submitted. */
>>> 	if (!bio) {
>>> 		if (!ret)
>>> 			folio_mark_uptodate(folio);
>>> 		folio_unlock(folio);
>>>
>>>                  ^^^^^^^^^^^^
>>>
>>> If all folios in rac have not been mapped (hole case), will we unlock the folio twice?
> 
> Are you worried the folio could be unlocked once in the if (rac) { ... } block and then
> unlocked again at err_out:? If so, I think that won't happen.
> 
> In such a case, every non-NULL folio will be unlocked exactly once by:
> 
> if (!folio_in_bio) {
>         if (!ret)
>                 folio_mark_uptodate(folio);
>         folio_unlock(folio);
> }
> Specifically, after the last folio runs through the block above, the next call:
> 
> folio = readahead_folio(rac);
> will return NULL. Then we go to next_folio:, and will directly hit:
> 
> if (!folio)
>         goto out;
> This jumps straight to the out: label, skipping err_out: entirely.
> Therefore, when ret is not an error code, the err_out: label will never be reached.
> 
> If ret becomes an error code, then the current folio will immediately goto err_out;
> and be unlocked there once.
> 
> If rac is NULL (meaning we only read the single large folio passed in as the function argument),
> we won't enter the if (rac) { ... goto next_folio; } path at all, so we also won't go to next_folio
> and then potentially goto out;. In that case, it will naturally be unlocked once at err_out:.
> Or am I missing some edge case here?

Nanzhe,

Oh, yes, I think so, thanks for the explanation.

Thanks,

> 
> Thanks,
> Nanzhe