linux-kernel - Re: [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online

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Date:   Tue, 8 Jun 2021 12:42:27 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online_mems() to get
 stable zone's values

On Mon, Jun 07, 2021 at 12:23:25PM +0200, Oscar Salvador wrote:
> I can see arguments for both riping it out and doing it right (but none for
> the way it is right now).
> For riping it out, one could say that those races might not be fatal,
> as usually the pfn you're working with (the one you want to check falls
> within a certain range) you know is valid, so the worst can happen is
> you get false positives/negatives and that might or might not be detected
> further down. How bad are false positive/negatives I guess it depends on the
> situation, but we already do that right now.
> The zone_spans_pfn() from page_outside_zone_boundaries() is the only one using
> locking right now, so well, if we survided this long without locks in other places
> using zone_spans_pfn() makes one wonder if it is that bad.

Givne that 

a) all current users of bad_range() are coming from VM_BUG_ON* callers
b) we only care when removing memory as the page would not lie in the
   zone anymore. But for that to happen the whole offline_pages() operation
   needs to succeed. bad_range() is called from rmqueue(), __free_one_page()
   and expand(). If offline_pages() succeeds for the range our page lies on,
   we would not be doing those operations on that page anyway?

So, I cannot find any strong reason to keep the seqlock (maybe in the future
we need to re-add it because some usecase).

Any objection on removing it?


-- 
Oscar Salvador
SUSE L3