On 1/14/20 8:28 PM, Mike Kravetz wrote:
On 1/14/20 5:24 PM, Yang Shi wrote:
On 1/14/20 5:07 PM, Mike Kravetz wrote:
On 1/14/20 6:09 AM, Li Xinhai wrote:
Add cc to
Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
, who has been worked on this part
On 2020-01-14 at 17:16 Li Xinhai wrote:
Checking MPOL_MF_STRICT is ignored for HUGETLB vma according to mbind man
page:
Notes
MPOL_MF_STRICT is ignored on huge page mappings.
If MPOL_MF_STRICT is specified alone without any MOVE flag, we should
indicate, from test_walk, that walking this vma should be skipped even if
there are misplaced pages.
Signed-off-by: Li Xinhai <lixinhai.lxh@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
I do not necessarily disagree with the change. However, this has made me
question a couple things:
1) Why does the man page say MPOL_MF_STRICT is ignored on huge page mappings?
- Is that leftover from the the days when huge page migration was not
supported?
- Is it just because huge page migration is more likely to fail than
base page migration.
2) Does the mbind code function properly when unable to migrate a huge page
MPOL_MF_STRICT is set? A quick look at the code looks like it returns
EIO.
I don't know the answer about question #1 I didn't dig into the history. The queue_pages_hugetlb() returns 0 unconditionally, I think this is what "MPOL_MF_STRICT is ignored on huge page mappings" means in code.
It would return -EIO for base pages or THP as what the manpage describes.
I was thinking about a migration failure after isolation. This block of
code in do_mbind() after queue_pages_range() and mbind_range().
if (!err) {
int nr_failed = 0;
if (!list_empty(&pagelist)) {
WARN_ON_ONCE(flags & MPOL_MF_LAZY);
nr_failed = migrate_pages(&pagelist, new_page, NULL,
start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND);
if (nr_failed)
putback_movable_pages(&pagelist);
}
if ((ret > 0) || (nr_failed && (flags & MPOL_MF_STRICT)))
err = -EIO;
Hmm.. I agree this part in man page does look ambiguous. We may assume
"MPOL_MF_STRICT is ignored on huge page mappings." implies if
MPOL_MF_STRICT is specified alone? If MOVE flag is specified it should
return -EIO if some pages could not be moved as what the man page describes.
I don't know what the intention was at the first place. We may have to
dig into the history.
