Messages in this thread | | | Date | Wed, 2 Jan 2008 21:31:36 +0100 | From | Christoph Hellwig <> | Subject | Re: getting rid of filp search in fs_may_remount_ro() |
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On Mon, Dec 31, 2007 at 11:54:26AM -0800, Dave Hansen wrote: > > Btw, I just noticed in current -mm fs_may_remount_ro() is still around > > and not replaced by ther per-sb writers count. That surely sounds like > > some kind of mismerge.. > > I was actually leaving that for later. Getting rid of the filp search > is a great benefit of the r/o bind patches, but it isn't strictly > necessary and it doesn't really hurt anything to keep it.
Well, it's one of the primary arguments for the expensive infrastructure behind the per-mount r/o patches, as checkin the counter here means we can now do exact tracking of the writable status of the superblock and thus avoid the remount r/o races.
But there's another argument for it which is unrelated to the per-mount r/o patches. fs_may_remount_ro is the last non-trivial user of sb->s_files and the file_list_lock. Peter Zijlstra identified the latter as scalabilty problem, and I'd really prefer to get rid of the list and the lock instead of hacking around these issues. Removing sb->s_files will also allow removing a list_head from struct file with some additional hackery to the tty core which should be a nice memory safer on big systems.
> The reason that it was contentious was that we need some way to be able > to do an sb-to-mount mapping. When remounting the sb, we need to > determine whether *any* of the mounts of that sb have any writers. > > We don't currently have any mechanisms to do direct lookups from sb to > mount. The only alternative I can see right now is to walk over all > tasks, then walk over all vfs namespaces, and walk each mount tree to > see if any mounts are of the sb we're looking for. This needs to be > done while already holding the mnt_writers[] locks so that no new mnt > writers can come in. > > *THAT* is going to be a heavyweight operation. I need to go look in > detail at how the mount trees are kept, and we'll need some kind of > mechanism to keep track of which vfs namespaces we've looked at during > the search so we don't search them twice. > > Can you think of a simpler way to do it?
Well, the first revisions of the patches added a list of vfsmounts to the superblock, and I wasn't quite happy with that because I though we could get around that easily. Given that we can't get around it easily I'd be happy to take my comments back.
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