From: Matthew Wilcox <willy@infradead.org> To: Andreas Gruenbacher <agruenba@redhat.com> Cc: linux-fsdevel@vger.kernel.org, cluster-devel@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jan Kara <jack@suse.cz> Subject: Re: [PATCH] [RFC] Trigger retry from fault vm operation Date: Tue, 11 May 2021 15:11:00 +0100 [thread overview] Message-ID: <YJqQdKmBHz6oEqD1@casper.infradead.org> (raw) In-Reply-To: <20210511140113.1225981-1-agruenba@redhat.com> On Tue, May 11, 2021 at 04:01:13PM +0200, Andreas Gruenbacher wrote: > we have a locking problem in gfs2 that I don't have a proper solution for, so > I'm looking for suggestions. > > What's happening is that a page fault triggers during a read or write > operation, while we're holding a glock (the cluster-wide gfs2 inode > lock), and the page fault requires another glock. We can recognize and > handle the case when both glocks are the same, but when the page fault requires > another glock, there is a chance that taking that other glock would deadlock. So we're looking at something like one file on a gfs2 filesystem being mmaped() and then doing read() or write() to another gfs2 file with the mmaped address being the passed to read()/write()? Have you looked at iov_iter_fault_in_readable() as a solution to your locking order? That way, you bring the mmaped page in first (see generic_perform_write()). > When we realize that we may not be able to take the other glock in gfs2_fault, > we need to communicate that to the read or write operation, which will then > drop and re-acquire the "outer" glock and retry. However, there doesn't seem > to be a good way to do that; we can only indicate that a page fault should fail > by returning VM_FAULT_SIGBUS or similar; that will then be mapped to -EFAULT. > We'd need something like VM_FAULT_RESTART that can be mapped to -EBUSY so that > we can tell the retry case apart from genuine -EFAULT errors. We do have VM_FAULT_RETRY ... does that retry at the wrong level?
WARNING: multiple messages have this Message-ID (diff)
From: Matthew Wilcox <willy@infradead.org> To: cluster-devel.redhat.com Subject: [Cluster-devel] [PATCH] [RFC] Trigger retry from fault vm operation Date: Tue, 11 May 2021 15:11:00 +0100 [thread overview] Message-ID: <YJqQdKmBHz6oEqD1@casper.infradead.org> (raw) In-Reply-To: <20210511140113.1225981-1-agruenba@redhat.com> On Tue, May 11, 2021 at 04:01:13PM +0200, Andreas Gruenbacher wrote: > we have a locking problem in gfs2 that I don't have a proper solution for, so > I'm looking for suggestions. > > What's happening is that a page fault triggers during a read or write > operation, while we're holding a glock (the cluster-wide gfs2 inode > lock), and the page fault requires another glock. We can recognize and > handle the case when both glocks are the same, but when the page fault requires > another glock, there is a chance that taking that other glock would deadlock. So we're looking at something like one file on a gfs2 filesystem being mmaped() and then doing read() or write() to another gfs2 file with the mmaped address being the passed to read()/write()? Have you looked at iov_iter_fault_in_readable() as a solution to your locking order? That way, you bring the mmaped page in first (see generic_perform_write()). > When we realize that we may not be able to take the other glock in gfs2_fault, > we need to communicate that to the read or write operation, which will then > drop and re-acquire the "outer" glock and retry. However, there doesn't seem > to be a good way to do that; we can only indicate that a page fault should fail > by returning VM_FAULT_SIGBUS or similar; that will then be mapped to -EFAULT. > We'd need something like VM_FAULT_RESTART that can be mapped to -EBUSY so that > we can tell the retry case apart from genuine -EFAULT errors. We do have VM_FAULT_RETRY ... does that retry at the wrong level?
next prev parent reply other threads:[~2021-05-11 14:11 UTC|newest] Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-11 14:01 [PATCH] [RFC] Trigger retry from fault vm operation Andreas Gruenbacher 2021-05-11 14:01 ` [Cluster-devel] " Andreas Gruenbacher 2021-05-11 14:11 ` Matthew Wilcox [this message] 2021-05-11 14:11 ` Matthew Wilcox 2021-05-11 14:59 ` Andreas Gruenbacher 2021-05-11 14:59 ` [Cluster-devel] " Andreas Gruenbacher 2021-05-11 14:59 ` Andreas Gruenbacher
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