Messages in this thread | | | Date | Tue, 3 Sep 2013 20:02:50 +0400 | From | Maxim Patlasov <> | Subject | Re: [PATCH 10/16] fuse: Implement writepages callback |
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09/03/2013 02:31 PM, Miklos Szeredi пишет: > On Fri, Aug 30, 2013 at 06:50:18PM +0400, Maxim Patlasov wrote: >> Hi Miklos, >> >> 08/30/2013 02:12 PM, Miklos Szeredi пишет: >>> On Fri, Aug 09, 2013 at 07:02:12PM +0400, Maxim Patlasov wrote: >>>> 08/06/2013 08:25 PM, Miklos Szeredi пишет: >>>>> Hmm. Direct IO on an mmaped file will do get_user_pages() which will >>>>> do the necessary page fault magic and ->page_mkwrite() will be called. >>>>> At least AFAICS. >>>> Yes, I agree. >>>> >>>>> The page cannot become dirty through a memory mapping without first >>>>> switching the pte from read-only to read-write first. Page accounting >>>>> logic relies on this too. The other way the page can become dirty is >>>>> through write(2) on the fs. But we do get notified about that too. >>>> Yes, that's correct, but I don't understand why you disregard two >>>> other cases of marking page dirty (both related to direct AIO read >>> >from a file to a memory region mmap-ed to a fuse file): >>>> 1. dio_bio_submit() --> >>>> bio_set_pages_dirty() --> >>>> set_page_dirty_lock() >>>> >>>> 2. dio_bio_complete() --> >>>> bio_check_pages_dirty() --> >>>> bio_dirty_fn() --> >>>> bio_set_pages_dirty() --> >>>> set_page_dirty_lock() >>>> >>>> As soon as a page became dirty through a memory mapping (exactly as >>>> you explained), nothing would prevent it to be written-back. And >>>> fuse will call end_page_writeback almost immediately after copying >>>> the real page to a temporary one. Then dio_bio_submit may re-dirty >>>> page speculatively w/o notifying fuse. And again, since then nothing >>>> would prevent it to be written-back once more. Hence we can end up >>>> in more then one temporary page in fuse write-back. And similar >>>> concern for dio_bio_complete() re-dirty. >>>> >>>> This make me think that we do need fuse_page_is_writeback() in >>>> fuse_writepages_fill(). But it shouldn't be harmful because it will >>>> no-op practically always due to waiting for fuse writeback in >>>> ->page_mkwrite() and in course of handling write(2). >>> The problem is: if we need it in ->writepages, we need it in ->writepage too. >>> And that's where we can't have it because it would deadlock in reclaim. >> I thought we're protected from the deadlock by the following chunk >> (in the very beginning of fuse_writepage): >> >>> + if (fuse_page_is_writeback(inode, page->index)) { >>> + if (wbc->sync_mode != WB_SYNC_ALL) { >>> + redirty_page_for_writepage(wbc, page); >>> + return 0; >>> + } >>> + fuse_wait_on_page_writeback(inode, page->index); >>> + } >> Because reclaimer will never call us with WB_SYNC_ALL. Did I miss >> something? > Yeah, we could have that in ->writepage() too. And apparently that would work, > reclaim would just leave us alone. > > Then there's sync(2) which does do WB_SYNC_ALL. Yet for an unprivileged fuse > mount we don't want ->writepages() to block because that's a quite clear DoS > issue.
Yes, I agree, but those cases (when sync(2) coincides with a page under fuse writeback originated by flusher coinciding with those direct AIO read redirty) should be very rare. I'd suggest to go on and put up with it for now: unprivileged users won't be able to use writeback_cache option until sysad enables allow_wbcache in fusermount.
> > So we are left with this:
Yes. May we implement it as a separate fix after inclusion of this patch-set?
> >>> There's a way to work around this: >>> >>> - if the request is still in queue, just update it with the contents of >>> the new page >>> >>> - if the request already in userspace, create a new reqest, but only let >>> userspace have it once the previous request for the same page >>> completes, so the ordering is not messed up >>> >>> But that's a lot of hairy code. >> Is it exactly how NFS solves similar problem? > NFS will apparently just block if there's a request outstanding and we are in > WB_SYNC_ALL mode. Which is somewhat simpler.
Yes, indeed.
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