2008/1/23, Miklos Szeredi <miklos@szeredi.hu>:
quoted text
> I think it would be more logical to move this patch forward, before
> the two patches which this document is actually describing.
>
> > Add a document, which describes how the POSIX requirements on updating
> > memory-mapped file times are addressed in Linux.
> >
> > Signed-off-by: Anton Salikhmetov <salikhmetov@gmail.com>
> > ---
> >  Documentation/vm/00-INDEX  |    2 +
> >  Documentation/vm/msync.txt |  117 ++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 119 insertions(+), 0 deletions(-)
> >
> > diff --git a/Documentation/vm/00-INDEX b/Documentation/vm/00-INDEX
> > index 2131b00..2726c8d 100644
> > --- a/Documentation/vm/00-INDEX
> > +++ b/Documentation/vm/00-INDEX
> > @@ -6,6 +6,8 @@ hugetlbpage.txt
> >       - a brief summary of hugetlbpage support in the Linux kernel.
> >  locking
> >       - info on how locking and synchronization is done in the Linux vm code.
> > +msync.txt
> > +     - the design document for memory-mapped file times update
> >  numa
> >       - information about NUMA specific code in the Linux vm.
> >  numa_memory_policy.txt
> > diff --git a/Documentation/vm/msync.txt b/Documentation/vm/msync.txt
> > new file mode 100644
> > index 0000000..571a766
> > --- /dev/null
> > +++ b/Documentation/vm/msync.txt
> > @@ -0,0 +1,117 @@
> > +
> > +     The msync() system call and memory-mapped file times
> > +
> > +     Copyright (C) 2008 Anton Salikhmetov
> > +
> > +The POSIX standard requires that any write reference to memory-mapped file
> > +data should result in updating the ctime and mtime for that file. Moreover,
> > +the standard mandates that updated file times should become visible to the
> > +world no later than at the next call to msync().
> > +
> > +Failure to meet this requirement creates difficulties for certain classes
> > +of important applications. For instance, database backup systems fail to
> > +pick up the files modified via the mmap() interface. Also, this is a
> > +security hole, which allows forging file data in such a manner that proving
> > +the fact that file data was modified is not possible.
> > +
> > +Briefly put, this requirement can be stated as follows:
> > +
> > +     once the file data has changed, the operating system
> > +     should acknowledge this fact by updating file metadata.
> > +
> > +This document describes how this POSIX requirement is addressed in Linux.
> > +
> > +1. Requirements
> > +
> > +1.1) the POSIX standard requires updating ctime and mtime not later
> > +than at the call to msync() with MS_SYNC or MS_ASYNC flags;
> > +
> > +1.2) in existing POSIX implementations, ctime and mtime
> > +get updated not later than at the call to fsync();
> > +
> > +1.3) in existing POSIX implementation, ctime and mtime
> > +get updated not later than at the call to sync(), the "auto-update" feature;
> > +
> > +1.4) the customers require and the common sense suggests that
> > +ctime and mtime should be updated not later than at the call to munmap()
> > +or exit(), the latter function implying an implicit call to munmap();
> > +
> > +1.5) the (1.1) item should be satisfied if the file is a block device
> > +special file;
> > +
> > +1.6) the (1.1) item should be satisfied for files residing on
> > +memory-backed filesystems such as tmpfs, too.
> > +
> > +The following operating systems were used as the reference platforms
> > +and are referred to as the "existing implementations" above:
> > +HP-UX B.11.31 and FreeBSD 6.2-RELEASE.
> > +
> > +2. Lazy update
> > +
> > +Many attempts before the current version implemented the "lazy update" approach
> > +to satisfying the requirements given above. Within the latter approach, ctime
> > +and mtime get updated at last moment allowable.
> > +
> > +Since we don't update the file times immediately, some Flag has to be
> > +used. When up, this Flag means that the file data was modified and
> > +the file times need to be updated as soon as possible.
> > +
> > +Any existing "dirty" flag which, when up, mean that a page has been written to,
> > +is not suitable for this purpose. Indeed, msync() called with MS_ASYNC
> > +would have to reset this "dirty" flag after updating ctime and mtime.
> > +The sys_msync() function itself is basically a no-op in the MS_ASYNC case.
> > +Thereby, the synchronization routines relying upon this "dirty" flag
> > +would lose data. Therefore, a new Flag has to be introduced.
> > +
> > +The (1.5) item coupled with (1.3) requirement leads to hard work with
> > +the block device inodes. Specifically, during writeback it is impossible to
> > +tell which block device file was originally mapped. Therefore, we need to
> > +traverse the list of "active" devices associated with the block device inode.
> > +This would lead to updating file times for block device files, which were not
> > +taking part in the data transfer.
> > +
> > +Also all versions prior to version 6 failed to correctly process ctime and
> > +mtime for files on the memory-backed filesystems such as tmpfs. So the (1.6)
> > +requirement was not satisfied.
>
> Version -8 also fails: for ram backed filesystems page tables are not
> write protected initially, nor after a sync.  This patch does write
> protect them after an MS_ASYNC, but that's a bug in the current
> context.

I've already written in the cover letter that functional tests passed
successfully.
In particular, this means that everything is OK with tmpfs too:

debian:~/times# ./times /mnt/file
begin   1201084493      1201084493      1201084281
write   1201084494      1201084494      1201084281
mmap    1201084494      1201084494      1201084495
b       1201084496      1201084496      1201084495
msync b 1201084496      1201084496      1201084495
c       1201084498      1201084498      1201084495
msync c 1201084498      1201084498      1201084495
d       1201084500      1201084500      1201084495
munmap  1201084500      1201084500      1201084495
close   1201084500      1201084500      1201084495
sync    1201084500      1201084500      1201084495
debian:~/times# mount | grep mnt
tmpfs on /mnt type tmpfs (rw)
debian:~/times#

quoted text
>
> > +
> > +If a write reference has occurred between two consecutive calls to msync()
> > +with MS_ASYNC, the second call to the latter function should take into
> > +account the last write reference. The last write reference can not be caught
> > +if no pagefault occurs. Hence a pagefault needs to be forced. This can be done
> > +using two different approaches. The first one is to synchronize data even when
> > +msync() was called with MS_ASYNC. This is not acceptable because the current
> > +design of the sys_msync() routine forbids starting I/O for the MS_ASYNC case.
>
> I don't think anyone forbids starting I/O, it's just too expensive,
> especially if it means, waiting for previous writeback on page to
> finish first.
>
> > +The second approach is to write protect the page for triggering a pagefault
> > +at the next write reference. Note that the dirty flag for the page should not
> > +be cleared thereby.
> > +
> > +In the "lazy update" approach, the requirements (1.1), (1.2), (1.3), and (1.4)
> > +taken together result in adding code at least to the following kernel routines:
> > +sys_msync(), do_fsync(), some routine in the unmap() call path, some routine
> > +in the sync() call path.
> > +
> > +Finally, a file_update_time()-like function would have to be created for
> > +processing the inode objects, not file objects. This is due to the fact that
> > +during the sync() operation, the file object may not exist any more, only
> > +the inode is known.
> > +
> > +To sum up: this "lazy" approach leads to massive changes, incurs overhead in
> > +the block device case, and requires complicated design decisions.
> > +
> > +3. Immediate update
> > +
> > +OK, still reading? There's a better way.
> > +
> > +In a fashion analogous to what happens at write(2), react to the fact
> > +that the page gets dirtied by updating the file times immediately.
> > +Thereby any page writeback happens when the write reference has already
> > +been accounted for from the view point of file times.
> > +
> > +The only problem which remains is to force refreshing file times at the write
> > +reference following a call to msync() with MS_ASYNC. As mentioned above, all
> > +that is needed here is to force a pagefault.
> > +
> > +The vma_wrprotect() routine introduced in this patch series is called
> > +from sys_msync() in the MS_ASYNC case. The former routine is essentially
> > +a version of existing page_mkclean_one() function from mm/rmap.c. Unlike
> > +the latter function, the vma_wrprotect() does not touch the dirty bit.
>
> Benchmark results should be also added to the relevant sections, I
> think.  There is a very definite cost to all this, and a 10x slowdown
> is usually not taken lightly...
>
> Great document, btw :)
>
> Miklos
>
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