Re: [PATCH v4 3/3] squashfs: implement readahead

[Marek Szyprowski]

Hi Matthew,

On 03.06.2022 17:29, Matthew Wilcox wrote:
> On Fri, Jun 03, 2022 at 10:55:01PM +0800, Hsin-Yi Wang wrote:
>> On Fri, Jun 3, 2022 at 10:10 PM Marek Szyprowski
>> <m.szyprowski@xxxxxxxxxxx> wrote:
>>> Hi Matthew,
>>>
>>> On 03.06.2022 14:59, Matthew Wilcox wrote:
>>>> On Fri, Jun 03, 2022 at 02:54:21PM +0200, Marek Szyprowski wrote:
>>>>> On 01.06.2022 12:39, Hsin-Yi Wang wrote:
>>>>>> Implement readahead callback for squashfs. It will read datablocks
>>>>>> which cover pages in readahead request. For a few cases it will
>>>>>> not mark page as uptodate, including:
>>>>>> - file end is 0.
>>>>>> - zero filled blocks.
>>>>>> - current batch of pages isn't in the same datablock or not enough in a
>>>>>>      datablock.
>>>>>> - decompressor error.
>>>>>> Otherwise pages will be marked as uptodate. The unhandled pages will be
>>>>>> updated by readpage later.
>>>>>>
>>>>>> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>>>>>> Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
>>>>>> Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>>>>>> Reported-by: Phillip Lougher <phillip@xxxxxxxxxxxxxxx>
>>>>>> Reported-by: Xiongwei Song <Xiongwei.Song@xxxxxxxxxxxxx>
>>>>>> ---
>>>>> This patch landed recently in linux-next as commit 95f7a26191de
>>>>> ("squashfs: implement readahead"). I've noticed that it causes serious
>>>>> issues on my test systems (various ARM 32bit and 64bit based boards).
>>>>> The easiest way to observe is udev timeout 'waiting for /dev to be fully
>>>>> populated' and prolonged booting time. I'm using squashfs for deploying
>>>>> kernel modules via initrd. Reverting aeefca9dfae7 & 95f7a26191deon on
>>>>> top of the next-20220603 fixes the issue.
>>>> How large are these files?  Just a few kilobytes?
>>> Yes, they are small, most of them are smaller than 16KB, some about
>>> 128KB and a few about 256KB. I've sent a detailed list in private mail.
>>>
>> Hi Marek,
>>
>> Are there any obvious squashfs errors in dmesg? Did you enable
>> CONFIG_SQUASHFS_FILE_DIRECT or CONFIG_SQUASHFS_FILE_CACHE?
> I don't think it's an error problem.  I think it's a short file problem.
>
> As I understand the current code (and apologies for not keeping up
> to date with how the patch is progressing), if the file is less than
> msblk->block_size bytes, we'll leave all the pages as !uptodate, leaving
> them to be brough uptodate by squashfs_read_folio().  So Marek is hitting
> the worst case scenario where we re-read the entire block for each page
> in it.  I think we have to handle this tail case in ->readahead().

I'm not sure if this is related to reading of small files. There are 
only 50 modules being loaded from squashfs volume. I did a quick test of 
reading the files.

Simple file read with this patch:

root@target:~# time find /initrd/ -type f | while read f; do cat $f 
 >/dev/null; done

real    0m5.865s
user    0m2.362s
sys     0m3.844s

Without:

root@target:~# time find /initrd/ -type f | while read f; do cat $f 
 >/dev/null; done

real    0m6.619s
user    0m2.112s
sys     0m4.827s

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland