Re: [PATCH][RFC][BUG] updating the ctime and mtime time stamps in msync()

Peter Staubach
2008-01-09T21:19:19Z

Anton Salikhmetov wrote:
> From: Anton Salikhmetov <salikhmetov@gmail.com>
>
> I would like to propose my solution for the bug #2645 from the kernel
bug tracker:
>
> http://bugzilla.kernel.org/show_bug.cgi?id=2645
>
> The Open Group defines the behavior of the mmap() function as follows.
>
> The st_ctime and st_mtime fields of a file that is mapped with MAP_SHARED
> and PROT_WRITE shall be marked for update at some point in the interval
> between a write reference to the mapped region and the next call to
msync()
> with MS_ASYNC or MS_SYNC for that portion of the file by any process.
> If there is no such call and if the underlying file is modified as a
result
> of a write reference, then these fields shall be marked for update at
some
> time after the write reference.
>
> The above citation was taken from the following link:
>
> http://www.opengroup.org/onlinepubs/009695399/functions/mmap.html
>
> Therefore, the msync() function should be called before verifying the
time
> stamps st_mtime and st_ctime in the test program Badari wrote in the
context
> of the bug #2645. Otherwise, the time stamps may be updated
> at some unspecified moment according to the POSIX standard.
>
> I changed his test program a little. The changed unit test can be
downloaded
> using the following link:
>
> http://pygx.sourceforge.net/mmap.c
>
> This program showed that the msync() function had a bug:
> it did not update the st_mtime and st_ctime fields.
>
> The program shows the appropriate behavior of the msync()
> function using the kernel with the proposed patch applied.
> Specifically, the ctime and mtime time stamps do change
> when modifying the mapped memory and do not change when
> there have been no write references between the mmap()
> and msync() system calls.
>
>

Sorry, I don't see where the test program shows that the file
times did not change if there had not been an intervening
modification to the mmap'd region. It appears to me that it
just shows the file times changing or not when there has been
intervening modification after the mmap call and before the
fstat call.

Or am I looking in the wrong place? :-)

> Additionally, the test cases for the msync() system call from
> the LTP test suite (msync01 - msync05, mmapstress01, mmapstress09,
> and mmapstress10) successfully passed using the kernel
> with the patch included into this email.
>
> The patch adds a call to the file_update_time() function to change
> the file metadata before syncing. The patch also contains
> substantial code cleanup: consolidated error check
> for function parameters, using the PAGE_ALIGN() macro instead of
> "manual" alignment check, improved readability of the loop,
> which traverses the process memory regions, updated comments.
>
>

These changes catch the simple case, where the file is mmap'd,
modified via the mmap'd region, and then an msync is done,
all on a mostly quiet system.

However, I don't see how they will work if there has been
something like a sync(2) done after the mmap'd region is
modified and the msync call. When the inode is written out
as part of the sync process, I_DIRTY_PAGES will be cleared,
thus causing a miss in this code.

The I_DIRTY_PAGES check here is good, but I think that there
needs to be some code elsewhere too, to catch the case where
I_DIRTY_PAGES is being cleared, but the time fields still need
to be updated.

--

A better architecture would be to arrange for the file times
to be updated when the page makes the transition from being
unmodified to modified. This is not straightforward due to
the current locking, but should be doable, I think. Perhaps
recording the current time and then using it to update the
file times at a more suitable time (no pun intended) might
work.

Thanx...

ps

> Signed-off-by: Anton Salikhmetov <salikhmetov@gmail.com>
>
> ---
>
> diff --git a/mm/msync.c b/mm/msync.c
> index 144a757..cb973eb 100644
> --- a/mm/msync.c
> +++ b/mm/msync.c
> @@ -1,26 +1,32 @@
> /*
> * linux/mm/msync.c
> *
> + * The msync() system call.
> * Copyright (C) 1994-1999 Linus Torvalds
> + *
> + * Updating the mtime and ctime stamps for mapped files
> + * and code cleanup.
> + * Copyright (C) 2008 Anton Salikhmetov <salikhmetov@gmail.com>
> */
>
> -/*
> - * The msync() system call.
> - */
> +#include <linux/file.h>
> #include <linux/fs.h>
> #include <linux/mm.h>
> #include <linux/mman.h>
> -#include <linux/file.h>
> -#include <linux/syscalls.h>
> #include <linux/sched.h>
> +#include <linux/syscalls.h>
>
> /*
> * MS_SYNC syncs the entire file - including mappings.
> *
> * MS_ASYNC does not start I/O (it used to, up to 2.5.67).
> - * Nor does it marks the relevant pages dirty (it used to up to 2.6.17).
> + * Nor does it mark the relevant pages dirty (it used to up to 2.6.17).
> * Now it doesn't do anything, since dirty pages are properly tracked.
> *
> + * The msync() system call updates the ctime and mtime fields for
> + * the mapped file when called with the MS_SYNC or MS_ASYNC flags
> + * according to the POSIX standard.
> + *
> * The application may now run fsync() to
> * write out the dirty pages and wait on the writeout and check the
result.
> * Or the application may run fadvise(FADV_DONTNEED) against the fd
to start
> @@ -33,70 +39,68 @@ asmlinkage long sys_msync(unsigned long start,
size_t len, int flags)
> unsigned long end;
> struct mm_struct *mm = current->mm;
> struct vm_area_struct *vma;
> - int unmapped_error = 0;
> - int error = -EINVAL;
> + int error = 0, unmapped_error = 0;
>
> - if (flags & ~(MS_ASYNC | MS_INVALIDATE | MS_SYNC))
> - goto out;
> - if (start & ~PAGE_MASK)
> + if ((flags & ~(MS_ASYNC | MS_INVALIDATE | MS_SYNC)) ||
> + (start & ~PAGE_MASK) ||
> + ((flags & MS_ASYNC) && (flags & MS_SYNC))) {
> + error = -EINVAL;
> goto out;
> - if ((flags & MS_ASYNC) && (flags & MS_SYNC))
> - goto out;
> - error = -ENOMEM;
> - len = (len + ~PAGE_MASK) & PAGE_MASK;
> + }
> +
> + len = PAGE_ALIGN(len);
> end = start + len;
> - if (end < start)
> + if (end < start) {
> + error = -ENOMEM;
> goto out;
> - error = 0;
> + }
> if (end == start)
> goto out;
> +
> /*
> * If the interval [start,end) covers some unmapped address ranges,
> * just ignore them, but return -ENOMEM at the end.
> */
> down_read(&mm->mmap_sem);
> vma = find_vma(mm, start);
> - for (;;) {
> + do {
> struct file *file;
>
> - /* Still start < end. */
> - error = -ENOMEM;
> - if (!vma)
> - goto out_unlock;
> - /* Here start < vma->vm_end. */
> + if (!vma) {
> + error = -ENOMEM;
> + break;
> + }
> if (start < vma->vm_start) {
> start = vma->vm_start;
> - if (start >= end)
> - goto out_unlock;
> + if (start >= end) {
> + error = -ENOMEM;
> + break;
> + }
> unmapped_error = -ENOMEM;
> }
> - /* Here vma->vm_start <= start < vma->vm_end. */
> if ((flags & MS_INVALIDATE) &&
> (vma->vm_flags & VM_LOCKED)) {
> error = -EBUSY;
> - goto out_unlock;
> + break;
> }
> file = vma->vm_file;
> - start = vma->vm_end;
> - if ((flags & MS_SYNC) && file &&
> - (vma->vm_flags & VM_SHARED)) {
> + if (file && (vma->vm_flags & VM_SHARED)) {
> get_file(file);
> - up_read(&mm->mmap_sem);
> - error = do_fsync(file, 0);
> - fput(file);
> - if (error || start >= end)
> - goto out;
> - down_read(&mm->mmap_sem);
> - vma = find_vma(mm, start);
> - } else {
> - if (start >= end) {
> - error = 0;
> - goto out_unlock;
> + if (file->f_mapping->host->i_state & I_DIRTY_PAGES)
> + file_update_time(file);
> + if (flags & MS_SYNC) {
> + up_read(&mm->mmap_sem);
> + error = do_fsync(file, 0);
> + down_read(&mm->mmap_sem);
> }
> - vma = vma->vm_next;
> + fput(file);
> + if (error)
> + break;
> }
> - }
> -out_unlock:
> +
> + start = vma->vm_end;
> + vma = vma->vm_next;
> + } while (start < end);
> up_read(&mm->mmap_sem);
> out:
> return error ? : unmapped_error;
>
> --
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