On (06/02/16 21:58), Ebru Akagunduz wrote:
[..]
> > I think it's this patch:
> >
> > http://ozlabs.org/~akpm/mmots/broken-out/mm-thp-make-swapin-readahead-under-down_read-of-mmap_sem.patch
> >
> > Some parts of the code in collapse_huge_page() that were under
> > down_write(mmap_sem) are under down_read() after the patch. But
> > there's "goto out" which continues via "goto out_up_write" which
> > does up_write(mmap_sem) so there's an imbalance. One path seems to
> > go via both up_read() and up_write(). I can imagine this can cause a
> > stuck down_write() among other things?
> Recently, I realized the same imbalance, it is an obvious
> inconsistency. I don't know, this issue can be related with
> mine. I'll send a fix patch.
a good find by Vlastimil.
Ebru, can you also re-visit __collapse_huge_page_swapin()? it's called
from collapse_huge_page() under the down_read(&mm->mmap_sem), is there
any reason to do the nested down_read(&mm->mmap_sem)?
collapse_huge_page()
...
down_read(&mm->mmap_sem);
result = hugepage_vma_revalidate(mm, vma, address);
if (result)
goto out;
pmd = mm_find_pmd(mm, address);
if (!pmd) {
result = SCAN_PMD_NULL;
goto out;
}
if (allocstall == curr_allocstall && swap != 0) {
if (!__collapse_huge_page_swapin(mm, vma, address, pmd)) {
{
: if (ret & VM_FAULT_RETRY) {
: down_read(&mm->mmap_sem);
: ^^^^^^^^^
: if (hugepage_vma_revalidate(mm, vma,
address))
: return false;
: }
}
up_read(&mm->mmap_sem);
goto out;
}
}
up_read(&mm->mmap_sem);
so if __collapse_huge_page_swapin() retruns true we have:
- down_read() twice, up_read() once?
the locking rules here are a bit confusing. (I didn't have my morning coffee
yet).
-ss
