Messages in this thread | | | Subject | Re: [GIT PULL] Please pull NFS client bugfixes.... | From | Trond Myklebust <> | Date | Thu, 07 Jan 2010 20:22:32 -0500 |
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On Thu, 2010-01-07 at 17:12 -0800, Linus Torvalds wrote: > > On Thu, 7 Jan 2010, Trond Myklebust wrote: > > > > > > Because it means that you can trivially take page faults before the thing > > > is validated (think threads). > > > > Which would mean that another process/thread already has part of the > > file mmapped on the same client. I'm not arguing that have to revalidate > > in _that_ case. > > No, I'm talking about the new mapping. Nothing else. > > If the mmap'ing thread releases mmap_sem, and then does the revalidate, > then you can have > > thread1 thread2 > ------- ------- > > mmap > map it in > release mmap_sem > page-fault the mapping before it got validated > ->post_mmap() > revalidate outside mmap_sem > > See? No "already part of the file mmapped" case at all. The exact mmap > that you just set up - without the revalidation having happened. > > In fact, because of this kind of _fundamental_ race, I don't see why I > would ever accept any patches that add multiple mmap() down-calls at > different phases to the filesystem at the VFS layer. > > A filesystem that depends on the different phases would be a fundamentally > buggy filesystem. Right now mmap is "atomic", and you can pre-populate (or > pre-verify, like NFS does) the mapping in the _knowledge_ that there are > no page faults that will populate it concurrently. Exactly because we hold > the mmap_sem for writing.
I don't think anyone has been advocating doing the revalidation _after_ the call to mmap_region(). All I want is to be able to do it as part of the mmap() syscall. It would be quite OK to add a ->pre_mmap() (which is what I believe Peter's patches do).
All I want to ensure is that people who use non-posix-lock based synchronisation can set the 'noac' flag, and be assured that if mmap() is called _after_ they have grabbed their lock, then the page cache will be duly revalidated (under the lock), and the fresh data will be made available.
Trond
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