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

[Matthew Wilcox]

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.

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.