Messages in this thread | | | Date | Sun, 5 Oct 2008 20:04:43 -0400 (EDT) | From | Mikulas Patocka <> | Subject | Re: application syncing options (was Re: [PATCH] Memory management livelock) |
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On Fri, 3 Oct 2008, david@lang.hm wrote:
> On Fri, 3 Oct 2008, Nick Piggin wrote: > > > > *What* is, forever? Data integrity syncs should have pages operated on > > > in-order, until we get to the end of the range. Circular writeback could > > > go through again, possibly, but no more than once. > > > > OK, I have been able to reproduce it somewhat. It is not a livelock, > > but what is happening is that direct IO read basically does an fsync > > on the file before performing the IO. The fsync gets stuck behind the > > dd that is dirtying the pages, and ends up following behind it and > > doing all its IO for it. > > > > The following patch avoids the issue for direct IO, by using the range > > syncs rather than trying to sync the whole file. > > > > The underlying problem I guess is unchanged. Is it really a problem, > > though? The way I'd love to solve it is actually by adding another bit > > or two to the pagecache radix tree, that can be used to transiently tag > > the tree for future operations. That way we could record the dirty and > > writeback pages up front, and then only bother with operating on them. > > > > That's *if* it really is a problem. I don't have much pity for someone > > doing buffered IO and direct IO to the same pages of the same file :) > > I've seen lots of discussions here about different options in syncing. in this > case a fix is to do a fsync of a range.
It fixes the bug in concurrent direct read+buffed write, but won't fix the bug with concurrent sync+buffered write.
> I've also seen discussions of how the > kernel filesystem code can do ordered writes without having to wait for them > with the use of barriers, is this capability exported to userspace? if so, > could you point me at documentation for it?
It isn't. And it is good that it isn't --- the more complicated API, the more maintenance work.
Mikulas
> David Lang >
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