Messages in this thread | | | Date | Thu, 24 May 2007 15:46:11 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 2/4] AFS: Add a function to excise a rejected write from the pagecache |
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On Thu, 24 May 2007 23:34:33 +0100 David Howells <dhowells@redhat.com> wrote:
> Andrew Morton <akpm@linux-foundation.org> wrote: > > > So my reason for asking the above is to try to find a way to make all these > > new PG-error games just go away. > > Yeah. However, there needs to be something to cover the gap between releasing > PG_writeback and getting PG_lock. They have to be done in that order to avoid > deadlocking against truncate and other stuff, but that leaves a window in which > the page appears to be in a good state - one in which prepare_write() or > page_mkwrite() can potentially leak through.
hm. I don't see why that race window would be a problem in practice: the page-exciser does a lock_page();wait_on_page_writeback() as normal, then proceeds with its business?
But given that this doesn't work right for some reason, can we use PG_error and then handle that appropriately in the filesystem's ->prepare_write() and ->page_mkwrite()?
> Nick Piggin talked about using an extra lock, but as far as I can tell, that > just compounds the deadlock problems. > > I suppose I could leave something in page->private that indicated that the > page was defunct, but that'd have to be done by the filesystem, probably > before calling cancel_rejected_write().
Well, using PG_error is OK and appropriate for that if it's localised to the fs. But I'd be a bit worried about requiring that the VFS maintain some special protocol for it, partly because it would be such a rarely-tested thing.
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