linux-kernel - Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

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Message-ID: <b8f36340-df56-1180-2a14-7b20cc1a0cda@oracle.com>
Date:   Tue, 13 Apr 2021 14:19:03 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...e.com>, Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

On 4/13/21 6:23 AM, Michal Hocko wrote:
> On Tue 13-04-21 12:47:43, Oscar Salvador wrote:
>> Currently, the clearing of the flag is done under the lock, but this
>> is unnecessary as we just allocated the page and we did not give it
>> away yet, so no one should be messing with it.
>>
>> Also, this helps making clear that here the lock is only protecting the
>> counter.
> 
> While moving the flag clearing is ok I am wondering why do we need that
> in the first place. I think it is just a leftover from 6c0371490140
> ("hugetlb: convert PageHugeFreed to HPageFreed flag"). Prior to that a tail
> page as been used to keep track of the state but now all happens in the
> head page and the flag uses page->private which is always initialized
> when allocated by the allocator (post_alloc_hook).

Yes, this was just left over from 6c0371490140.  And, as you mention
post_alloc_hook will clear page->private for all non-gigantic pages
allocated via buddy.

> Or do we need it for giga pages which are not allocated by the page
> allocator? If yes then moving it to prep_compound_gigantic_page would be
> better.

I am pretty sure dynamically allocated giga pages have page->Private
cleared as well.  It is not obvious, but the alloc_contig_range code
used to put together giga pages will end up calling isolate_freepages_range
that will call split_map_pages and then post_alloc_hook for each MAX_ORDER
block.  As mentioned, this is not obvious and I would hate to rely on this
behavior not changing.

> 
> So should we just drop it here?

The only place where page->private may not be initialized is when we do
allocations at boot time from memblock.  In this case, we will add the
pages to the free list via put_page/free_huge_page so the appropriate
flags will be cleared before anyone notices.

I'm wondering if we should just do a set_page_private(page, 0) here in
prep_new_huge_page since we now use that field for flags.  Or, is that
overkill?
-- 
Mike Kravetz