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charset=us-ascii Content-Disposition: inline In-Reply-To: <20200211122323.GS8731@bombadil.infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Feb 11, 2020 at 04:23:23AM -0800, Matthew Wilcox wrote: > On Mon, Feb 10, 2020 at 08:25:36PM -0800, Minchan Kim wrote: > > On Mon, Feb 10, 2020 at 07:54:12PM -0800, Matthew Wilcox wrote: > > > On Mon, Feb 10, 2020 at 07:50:04PM -0800, Minchan Kim wrote: > > > > On Mon, Feb 10, 2020 at 05:10:21PM -0800, Matthew Wilcox wrote: > > > > > On Mon, Feb 10, 2020 at 04:19:58PM -0800, Minchan Kim wrote: > > > > > > filemap_fault > > > > > > find a page form page(PG_uptodate|PG_readahead|PG_writeback) > > > > > > > > > > Uh ... That shouldn't be possible. > > > > > > > > Please see shrink_page_list. Vmscan uses PG_reclaim to accelerate > > > > page reclaim when the writeback is done so the page will have both > > > > flags at the same time and the PG reclaim could be regarded as > > > > PG_readahead in fault conext. > > > > > > What part of fault context can make that mistake? The snippet I quoted > > > below is from page_cache_async_readahead() where it will clearly not > > > make that mistake. There's a lot of code here; please don't presume I > > > know all the areas you're talking about. > > > > Sorry about being not clear. I am saying filemap_fault -> > > do_async_mmap_readahead > > > > Let's assume the page is hit in page cache and vmf->flags is !FAULT_FLAG > > TRIED so it calls do_async_mmap_readahead. Since the page has PG_reclaim > > and PG_writeback by shrink_page_list, it goes to > > > > do_async_mmap_readahead > > if (PageReadahead(page)) > > fpin = maybe_unlock_mmap_for_io(); > > page_cache_async_readahead > > if (PageWriteback(page)) > > return; > > ClearPageReadahead(page); <- doesn't reach here until the writeback is clear > > > > So, mm_populate will repeat the loop until the writeback is done. > > It's my just theory but didn't comfirm it by the testing. > > If I miss something clear, let me know it. > > Ah! Surely the right way to fix this is ... I'm not sure it's right fix. Actually, I wanted to remove PageWriteback check in page_cache_async_readahead because I don't see corelation. Why couldn't we do readahead if the marker page is PG_readahead|PG_writeback design PoV? Only reason I can think of is it makes *a page* will be delayed for freeing since we removed PG_reclaim bit, which would be over-optimization for me. Other concern is isn't it's racy? IOW, page was !PG_writeback at the check below in your snippet but it was under PG_writeback in page_cache_async_readahead and then the IO was done before refault reaching the code again. It could be repeated *theoretically* even though it's very hard to happen in real practice. Thus, I think it would be better to remove PageWriteback check from page_cache_async_readahead if we really want to go the approach. However, page_cache_async_readahead has another condition to bail out: ra_pages I think it's also racy with fadvise or shrinking the window size from other tasks. That's why I thought second trial with non-fault retry logic from caller would fix all potnetial issues all at once like page fault handler have done. > > +++ b/mm/filemap.c > @@ -2420,7 +2420,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, > return fpin; > if (ra->mmap_miss > 0) > ra->mmap_miss--; > - if (PageReadahead(page)) { > + if (!PageWriteback(page) && PageReadahead(page)) { > fpin = maybe_unlock_mmap_for_io(vmf, fpin); > page_cache_async_readahead(mapping, ra, file, > page, offset, ra->ra_pages); >