On Mon, Jan 23 2017, Jeff Layton wrote: > On Tue, 2017-01-24 at 11:16 +1100, NeilBrown wrote: >> On Mon, Jan 23 2017, Trond Myklebust wrote: >> >> > On Mon, 2017-01-23 at 17:35 -0500, Jeff Layton wrote: >> > > On Mon, 2017-01-23 at 11:09 +0100, Kevin Wolf wrote: >> > > > >> > > > However, if we look at the greater problem of hanging requests that >> > > > came >> > > > up in the more recent emails of this thread, it is only moved >> > > > rather >> > > > than solved. Chances are that already write() would hang now >> > > > instead of >> > > > only fsync(), but we still have a hard time dealing with this. >> > > > >> > > >> > > Well, it _is_ better with O_DIRECT as you can usually at least break >> > > out >> > > of the I/O with SIGKILL. >> > > >> > > When I last looked at this, the problem with buffered I/O was that >> > > you >> > > often end up waiting on page bits to clear (usually PG_writeback or >> > > PG_dirty), in non-killable sleeps for the most part. >> > > >> > > Maybe the fix here is as simple as changing that? >> > >> > At the risk of kicking off another O_PONIES discussion: Add an >> > open(O_TIMEOUT) flag that would let the kernel know that the >> > application is prepared to handle timeouts from operations such as >> > read(), write() and fsync(), then add an ioctl() or syscall to allow >> > said application to set the timeout value. >> >> I was thinking on very similar lines, though I'd use 'fcntl()' if >> possible because it would be a per-"file description" option. >> This would be a function of the page cache, and a filesystem wouldn't >> need to know about it at all. Once enable, 'read', 'write', or 'fsync' >> would return EWOULDBLOCK rather than waiting indefinitely. >> It might be nice if 'select' could then be used on page-cache file >> descriptors, but I think that is much harder. Support O_TIMEOUT would >> be a practical first step - if someone agreed to actually try to use it. >> > > Yeah, that does seem like it might be worth exploring. > > That said, I think there's something even simpler we can do to make > things better for a lot of cases, and it may even help pave the way for > the proposal above. > > Looking closer and remembering more, I think the main problem area when > the pages are stuck in writeback is the wait_on_page_writeback call in > places like wait_for_stable_page and __filemap_fdatawait_range. I can't see wait_for_stable_page() being very relevant. That only blocks on backing devices which have requested stable pages. raid5 sometimes does that. Some scsi/sata devices can somehow. And rbd (part of ceph) sometimes does. I don't think NFS ever will. wait_for_stable_page() doesn't currently return an error, so getting to abort in SIGKILL would be a lot of work. filemap_fdatawait_range() is much easier. diff --git a/mm/filemap.c b/mm/filemap.c index b772a33ef640..2773f6dde1da 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -401,7 +401,9 @@ static int __filemap_fdatawait_range(struct address_space *mapping, if (page->index > end) continue; - wait_on_page_writeback(page); + if (PageWriteback(page)) + if (wait_on_page_bit_killable(page, PG_writeback)) + err = -ERESTARTSYS; if (TestClearPageError(page)) ret = -EIO; } That isn't a complete solution. There is code in f2fs which doesn't check the return value and probably should. And gfs2 calls mapping_set_error(mapping, error); with the return value, with we probably don't want in the ERESTARTSYS case. There are some usages in btrfs that I'd need to double-check too. But it looks to be manageable. Thanks, NeilBrown > > That uses an uninterruptible sleep and it's common to see applications > stuck there in these situations. They're unkillable too so your only > recourse is to hard reset the box when you can't reestablish > connectivity. > > I think it might be good to consider making some of those sleeps > TASK_KILLABLE. For instance, both of the above callers of those > functions are int return functions. It may be possible to return > ERESTARTSYS when the task catches a signal. > > -- > Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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