Re: [PATCH v2] mm : sync ra->ra_pages with bdi->ra_pages

[Zhaoyang Huang]

On Fri, Aug 21, 2020 at 7:57 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Fri, Aug 21, 2020 at 05:31:52PM +0800, Zhaoyang Huang wrote:
> > This patch has been verified on an android system and reduces 15% of
> > UNITERRUPTIBLE_SLEEP_BLOCKIO which was used to be caused by wrong
> > ra->ra_pages.
>
> Wait, what?  Readahead doesn't sleep on the pages it's requesting.
> Unless ... your file access pattern is random, so you end up submitting
> a readahead I/O that's bigger than needed, so takes longer for the page
> you actually wanted to be returned.  I know we have the LOTSAMISS
> logic, but that's not really enough.
actually, async read even if hitting the marker will also introduce
huge mount of read by wrong ra->ra_pages, which will be used as
req_size for ondemand_readahead.
>
> OK, assuming this problem is really about sync mmap (ie executables),
> this makes a bit more sense.  I think the real problem is here:
>
>         ra->start = max_t(long, 0, offset - ra->ra_pages / 2);
>         ra->size = ra->ra_pages;
>         ra->async_size = ra->ra_pages / 4;
>         ra_submit(ra, mapping, file);
>
> which actually skips all the logic we have in ondemand_readahead()
> for adjusting the readahead size.  Ugh, this is a mess.
>
> I think a quick fix to your problem will be just replacing ra->ra_pages
> with bdi->ra_pages in do_sync_mmap_readahead() and leaving ra->ra_pages
> alone everywhere else.
>
> We need a smarter readahead algorithm for mmap'ed files, and I don't have
> time to work on it right now.  So let's stick to the same dumb algorithm,
> but make it responsive to bdi ra_pages being reset.