Re: [PATCH v2] btrfs: zlib: handle page aligned compressed size correctly

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2026/3/17 20:41, David Sterba 写道:
> On Fri, Mar 13, 2026 at 06:35:26PM +1030, Qu Wenruo wrote:
[...]
>> diff --git a/fs/btrfs/zlib.c b/fs/btrfs/zlib.c
>> index 1a5093525e32..147c92a4dd04 100644
>> --- a/fs/btrfs/zlib.c
>> +++ b/fs/btrfs/zlib.c
>> @@ -308,7 +308,9 @@ int zlib_compress_bio(struct list_head *ws, struct compressed_bio *cb)
>>   	}
>>   	/* Queue the remaining part of the folio. */
> 
> Maybe update this comment with brief description of the tricky case.

To be honest, the tricky case is not tricky at all.

It's myself considering large folios and bs > ps cases too much.

The new code is in fact more straightforward, if there are new output, 
total_out - bi_size is exactly the length of the new output.

And the output length is already limited by folio_size, there is no need 
and shouldn't use offset_in_folio().
The offset_in_folio() looks sane on a quick glance, but if you put all 
the corner values (0, and folio_size) into it, it only passes the 0 case 
but not the folio_size case.

So there is no tricky case in the first place, it's all my fault making 
things way too complex and shoot myself in the foot.

Thanks,
Qu


> 
>>   	if (workspace->strm.total_out > bio->bi_iter.bi_size) {
>> -		u32 cur_len = offset_in_folio(out_folio, workspace->strm.total_out);
>> +		const u32 cur_len = workspace->strm.total_out - bio->bi_iter.bi_size;
>> +
>> +		ASSERT(cur_len <= folio_size(out_folio));
>>   
>>   		if (!bio_add_folio(bio, out_folio, cur_len, 0)) {
>>   			ret = -E2BIG;
>> -- 
>> 2.53.0
>>
>