RE: Hugepage regression

[Chen, Kenneth W]

Hugh Dickins wrote on Tuesday, October 10, 2006 12:18 PM
> On Tue, 10 Oct 2006, Chen, Kenneth W wrote:
> > 
> > With the pending shared page table for hugetlb currently sitting in -mm,
> > we serialize the all hugetlb unmap with a per file i_mmap_lock.  This
> > race could well be solved by that pending patch?
> > 
> >
http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.19-rc1/2.6.19-rc1-mm1/broken-out/shared-page-table-for-hugetlb-page-v
4.patch
> 
> Hey, nice try, Ken!  But I don't think we can let you sneak shared
> pagetables into 2.6.19 that way ;)

It wasn't my intention to sneak in shared page table, though it does
sort of look like so.


> Yes, I'd expect your i_mmap_lock to solve the problem: and since
> you're headed in that direction anyway, it makes most sense to use
> that solution rather than get into defining arrays, or sacrificing
> the lazy flush, or risking page_count races.
> 
> So please extract the __unmap_hugepage_range mods from your shared
> pagetable patch, and use that to fix the bug.

OK, I was about to do so too.


> But again, I protest
> the "if (vma->vm_file)" in your unmap_hugepage_range - how would a
> hugepage area ever have NULL vma->vm_file?

It's coming from do_mmap_pgoff(), file->f_op->mmap can fail with error
code (e.g. not enough hugetlb page) and in the error recovery path, it
nulls out vma->vm_file first before calls down to unmap_region().  I
asked that question before: can we reverse that order (call unmap_region
and then nulls out vma->vmfile and fput)?

unmap_and_free_vma:
        if (correct_wcount)
                atomic_inc(&inode->i_writecount);
        vma->vm_file = NULL;
        fput(file);

        /* Undo any partial mapping done by a device driver. */
        unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);

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