Re: [PATCH] thp: close race between split and zap huge pages

[Andrea Arcangeli <aarcange@redhat.com>]

Hi everyone,

On Wed, Apr 16, 2014 at 12:48:56AM +0300, Kirill A. Shutemov wrote:
> -	pmd = mm_find_pmd(mm, address);
> -	if (!pmd)
> +	pgd = pgd_offset(mm, address);
> +	if (!pgd_present(*pgd))
>  		return NULL;
> +	pud = pud_offset(pgd, address);
> +	if (!pud_present(*pud))
> +		return NULL;
> +	pmd = pmd_offset(pud, address);

This fix looks good to me and it was another potential source of
trouble making the BUG_ON flakey. But the rmap_walk out of order
problem still exists too I think. Possibly the testcase doesn't
exercise that.

> -	if (pmd_none(*pmd))
> +	if (!pmd_present(*pmd))
>  		goto unlock;

pmd_present is a bit slower, but functionally it's equivalent, the
pmd_present check is just more pedantic (kind of defining the
invariants for how a mapped pmd should look like).

If we'd add native THP swapout later !pmd_present would be more
correct for the VM calls to page_check_address_pmd, but something
would need changing anyway if split_huge_page is the callee as I don't
think we can skip the conversion from trans huge swap entry to linear
swap entries and the pmd2pte conversion.

The main reason that most places that could run into a trans huge pmd
would use pmd_none and never pmd_present is that originally
pmd_present wouldn't check _PAGE_PSE and _PAGE_PRESENT can be
temporarily be cleared with pmdp_invalidate on trans huge pmds. Now
pmd_present is safe too so there's no problem in using it on trans
huge pmds.

So either pmd_none !pmd_present are fine, the functional fix is the
part above.

Thanks!
Andrea

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