On Fri 19-02-21 10:05:53, Oscar Salvador wrote:
> On Thu, Feb 18, 2021 at 02:59:40PM +0100, Michal Hocko wrote:
> > > It should be:
> > >
> > > allocate_a_new_page (new_page's refcount = 1)
> > > put_page(new_page); (new_page's refcount = 0)
> > > dissolve_old_page
> > > : if fail
> > > dissolve_new_page (we can dissolve it as refcount == 0)
> > >
> > > I hope this clarifies it .
> >
> > OK, I see the problem now. And your above solution is not really
> > optimal either. Your put_page would add the page to the pool and so it
> > could be used by somebody. One way around it would be either directly
> > manipulating reference count which is fugly or you can make it a
> > temporal page (alloc_migrate_huge_page) or maybe even better not special
> > case this here but rather allow migrating free hugetlb pages in the
> > migrate_page path.
>
> I have been weighting up this option because it seemed the most clean way to
> proceed. Having the hability to migrate free hugepages directly from migrate_page
> would spare us this function.
> But there is a problem. migrate_pages needs the pages to be on a list (by
> page->lru). That is no problem for used pages, but for freehugepages we would
> have to remove the page from hstate->hugepage_freelists, meaning that if userspace
> comes along and tries to get a hugepage (a hugepage he previously allocated by
> e.g: /proc/sys/.../nr_hugepages), it will fail.
Good point. I should have realized that.
> So I am not really sure we can go this way. Unless we are willing to accept
> that temporary userspace can get ENOMEM if it tries to use a hugepage, which
> I do not think it is a good idea.
No, this is not acceptable.
> Another way to go would be to make up for the free hugepages to be migrated and
> allocate the same amount, but that starts to go down a rabbit hole.
>
> I yet need to give it some more spins, but all in all, I think the easiest way
> forward way might be to do something like:
>
> alloc_and_dissolve_huge_page {
>
> new_page = alloc_fresh_huge_page(h, gfp_mask, nid, NULL, NULL);
> if (new_page) {
> /*
> * Put the page in the freelist hugepage pool.
> * We might race with someone coming by and grabbing the page,
> * but that is fine since we mark the page as Temporary in case
> * both old and new_page fail to be dissolved, so new_page
> * will be freed when its last reference is gone.
> */
> put_page(new_page);
>
> if (!dissolve_free_huge_page(page)) {
> /*
> * Old page could be dissolved.
> */
> ret = true;
> } else if (dissolve_free_huge_page(new_page)) {
> /*
> * Page might have been dissolved by admin by doing
> * "echo 0 > /proc/../nr_hugepages". Check it before marking
> * the page.
> */
> spin_lock(&hugetlb_lock);
> /* Mark the page Temporary in case we fail to dissolve both
> * the old page and new_page. It will be freed when the last
> * user drops it.
> */
> if (PageHuge(new_page))
> SetPageHugeTemporary(new_page);
> spin_unlock(&hugetlb_lock);
> }
> }
OK, this should work but I am really wondering whether it wouldn't be
just simpler to replace the old page by a new one in the free list
directly. Or is there any reason we have to go through the generic
helpers path? I mean something like this
new_page = alloc_fresh_huge_page();
if (!new_page)
goto fail;
spin_lock(hugetlb_lock);
if (!PageHuge(old_page)) {
/* freed from under us, nothing to do */
__update_and_free_page(new_page);
goto unlock;
}
list_del(&old_page->lru);
__update_and_free_page(old_page);
__enqueue_huge_page(new_page);
unlock:
spin_unlock(hugetlb_lock);
This will require to split update_and_free_page and enqueue_huge_page to
counters independent parts but that shouldn't be a big deal. But it will
also protect from any races. Not an act of beauty but seems less hackish
to me.
--
Michal Hocko
SUSE Labs
