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Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
From:   David Hildenbrand <david@redhat.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Oscar Salvador <osalvador@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Minchan Kim <minchan@kernel.org>, Jann Horn <jannh@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dave Hansen <dave.hansen@intel.com>,
        Hugh Dickins <hughd@google.com>,
        Rik van Riel <riel@surriel.com>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
        Helge Deller <deller@gmx.de>, Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>, linux-alpha@vger.kernel.org,
        linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
        linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org
References: <20210217154844.12392-1-david@redhat.com>
 <YC5Am6a4KMSA8XoK@dhcp22.suse.cz>
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Organization: Red Hat GmbH
Message-ID: <de28b8db-a103-5bc2-8ace-d2907026a95d@redhat.com>
Date:   Thu, 18 Feb 2021 11:54:48 +0100
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>>>      If we hit
>>>      hardware errors on pages, ignore them - nothing we really can or
>>>      should do.
>>> 3. On errors during MADV_POPULATED, some memory might have been
>>>      populated. Callers have to clean up if they care.
>>
>> How does caller find out? madvise reports 0 on success so how do you
>> find out how much has been populated?
> 
> If there is an error, something might have been populated. In my QEMU
> implementation, I simply discard the range again, good enough. I don't
> think we need to really indicate "error and populated" or "error and not
> populated".

Clarifying again: if madvise(MADV_POPULATED) succeeds, it returns 0. If 
there was a problem poopulating memory, it returns -ENOMEM (similar to 
MADV_WILLNEED). Callers can detect the error and discard.

-- 
Thanks,

David / dhildenb