Re: anon_vma RFC2

William Lee Irwin III
2004-03-14T02:27:59Z

On Sat, 13 Mar 2004, William Lee Irwin III wrote:
>> [hugetlb at same address]

On Sat, Mar 13, 2004 at 08:41:42PM -0500, Rik van Riel wrote:
> Well, we can find this merely by looking at the page tables
> themselves, so that shouldn't be a problem.

Pagetables of a kind the core understands may not be present there.
On ia32 one could in theory have a pmd_huge() check, which would in
turn not suffice for ia64 and sparc64 hugetlb. These were only examples.
Other unusual forms of mappings, e.g. VM_RESERVED and VM_IO, may also
be bad ideas to trip over by accident.

On Sat, 13 Mar 2004, William Lee Irwin III wrote:
>> Searching the COW sharing group isn't how everything works, but in those
>> cases where additionally you can find mm's that don't map the page at
>> that virtual address and may have different vmas cover it, this can
>> arise.

On Sat, Mar 13, 2004 at 08:41:42PM -0500, Rik van Riel wrote:
> This could only happen when you truncate a file that's
> been mapped by various nonlinear VMAs, so truncate can't
> get rid of the pages...
> I suspect there are two ways to fix that:
> 1) on truncate, scan ALL the ptes inside nonlinear VMAs
> and remove the pages
> 2) don't allow truncate on a file that's mapped with
> nonlinear VMAs
> Either would work.

I'm not sure how that came in. The issue I had in mind was strictly
a matter of tripping over things one can't make sense of from
pagetables alone in try_to_unmap().

COW-shared anonymous pages not unmappable via anonymous COW sharing
groups arising from truncate() vs. remap_file_pages() interactions and
failures to check for nonlinearly-mapped pages in pagetable walkers are
an issue in general of course, but they just aren't this issue.

-- wli