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SubjectRe: [PATCH] Fix vmtruncate race and distributed filesystem race
Hi,

On Thu, Jun 12, 2003 at 05:56:50PM -0500, Dave McCracken wrote:
> --- 2.5.70-mm8/./mm/memory.c 2003-06-12 13:37:31.000000000 -0500
> +++ 2.5.70-mm8-trunc/./mm/memory.c 2003-06-12 17:51:55.000000000 -0500
> @@ -1138,6 +1138,8 @@ invalidate_mmap_range(struct address_spa
> hlen = ULONG_MAX - hba + 1;
> }
> down(&mapping->i_shared_sem);
> + /* Protect against page fault */
> + atomic_inc(&mapping->truncate_count);
> if (unlikely(!list_empty(&mapping->i_mmap)))
> invalidate_mmap_range_list(&mapping->i_mmap, hba, hlen);
> if (unlikely(!list_empty(&mapping->i_mmap_shared)))
> @@ -1390,8 +1392,10 @@ do_no_page(struct mm_struct *mm, struct
> unsigned long address, int write_access, pte_t *page_table, pmd_t *pmd)
> {
> struct page * new_page;
> + struct address_space *mapping;
> pte_t entry;
> struct pte_chain *pte_chain;
> + unsigned sequence;
> int ret;
>
> if (!vma->vm_ops || !vma->vm_ops->nopage)
> @@ -1400,6 +1404,9 @@ do_no_page(struct mm_struct *mm, struct
> pte_unmap(page_table);
> spin_unlock(&mm->page_table_lock);
>
> + mapping = vma->vm_file->f_dentry->d_inode->i_mapping;
> +retry:
> + sequence = atomic_read(&mapping->truncate_count);
> new_page = vma->vm_ops->nopage(vma, address & PAGE_MASK, 0);
>
> /* no page was available -- either SIGBUS or OOM */
> @@ -1428,6 +1435,16 @@ do_no_page(struct mm_struct *mm, struct
> }
>
> spin_lock(&mm->page_table_lock);
> + /*
> + * For a file-backed vma, someone could have truncated or otherwise
> + * invalidated this page. If invalidate_mmap_range got called,
> + * retry getting the page.
> + */
> + if (unlikely(sequence != atomic_read(&mapping->truncate_count))) {
> + spin_unlock(&mm->page_table_lock);
> + page_cache_release(new_page);
> + goto retry;
> + }
> page_table = pte_offset_map(pmd, address);

maybe I'm missing something silly but this fixes nothing IMHO. It's not
a coincidence I used the seq_lock (aka frlock in 2.4-aa) in my fix, a
single counter increment isn't nearly enough, you definitely need _both_
an entry and exit point in do_truncate or you'll never know if
vmtruncate has been running under you. The first increment is like the
down_read, the second increment is the up_read. Both are necessary to
trap any vmtruncate during the do_no_page.

Your patch traps this timing case:

CPU 0 CPU 1
---------- -----------
do_no_page
truncate
read counter

increment counter
vmtruncate
->nopage
read counter again -> different so retry


but you can't trap this with a single counter increment in do_truncate:

CPU 0 CPU 1
---------- -----------
do_no_page
truncate
increment counter
read counter
->nopage
vmtruncate
read counter again -> different so retry

thanks to the second counter increment after vmtruncate in my fix, the
above race couldn't happen.

About the down(&inode->i_sem); up(), that you dropped under Andrew's
suggestion, while that maybe ugly, it will have a chance to save cpu,
and since it's a slow path such goto, it's definitely worthwhile to keep
it IMHO. Otherwise one cpu will keep scheduling in a loop until truncate
returns, and it can take time since it may have to do I/O or wait on
some I/O semaphore. It wouldn't be DoSable, because the
ret-from-exception will check need_resched, but still it's bad for cpu
utilization and such a waste can be avoided trivially as in my fix.

I was chatting with Daniel about those hooks a few minutes ago, and he
suggested to make do_no_page a callback itself (instead of having
do_no_page call into a ->nopage further callback). And to provide by
default a generic implementation that would be equivalent to the current
do_no_page. As far as I can tell that will save both the new pointer to
function for the DSM hook (that IMHO has to be taken twice, both before
->nopage and after ->nopage, not only before the ->nopage, for the
reason explained above) and the ->nopage hook itself. So maybe that
could be a cleaner solution to avoid the DSM hooks enterely, so we don't
have more hooks but less, and a library call. This sounds the best for
performance and flexibility. (talking only about 2.5 of course, 2.4 I
think is just fine with my ""production"" 8) fix here:

http://www.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.21rc8aa1/2.4.21rc8aa1/9999_truncate-nopage-race-1

)

Andrea
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