Re: [PATCH v1] madvise.2: Document MADV_POPULATE_READ and MADV_POPULATE_WRITE

From: David Hildenbrand <david@redhat.com>
To: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: linux-man@vger.kernel.org,
	Alejandro Colomar <alx.manpages@gmail.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>,
	Oscar Salvador <osalvador@suse.de>, Jann Horn <jannh@google.com>,
	Mike Rapoport <rppt@kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>
Subject: Re: [PATCH v1] madvise.2: Document MADV_POPULATE_READ and MADV_POPULATE_WRITE
Date: Mon, 12 Jul 2021 12:03:57 +0200	[thread overview]
Message-ID: <406695f0-13c8-a052-92d4-44e884eb44af@redhat.com> (raw)
In-Reply-To: <CAM9Jb+g7a5kY4DHzDzJTfs-mVEm_CFRcCeY2zdjo6sZWxkjnLA@mail.gmail.com>

On 12.07.21 11:58, Pankaj Gupta wrote:
>> MADV_POPULATE_READ and MADV_POPULATE_WRITE have been merged into
>> upstream Linux via commit 4ca9b3859dac ("mm/madvise: introduce
>> MADV_POPULATE_(READ|WRITE) to prefault page tables"), part of v5.14-rc1.
>>
>> Let's document the behavior and error conditions of these new madvise()
>> options.
>>
>> Cc: Alejandro Colomar <alx.manpages@gmail.com>
>> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Jann Horn <jannh@google.com>
>> Cc: Mike Rapoport <rppt@kernel.org>
>> Cc: Linux API <linux-api@vger.kernel.org>
>> Cc: linux-mm@kvack.org
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   man2/madvise.2 | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 80 insertions(+)
>>
>> diff --git a/man2/madvise.2 b/man2/madvise.2
>> index f1f384c0c..3ec8c53a7 100644
>> --- a/man2/madvise.2
>> +++ b/man2/madvise.2
>> @@ -469,6 +469,59 @@ If a page is file-backed and dirty, it will be written back to the backing
>>   storage.
>>   The advice might be ignored for some pages in the range when it is not
>>   applicable.
>> +.TP
>> +.BR MADV_POPULATE_READ " (since Linux 5.14)
>> +Populate (prefault) page tables readable for the whole range without actually
>> +reading. Depending on the underlying mapping, map the shared zeropage,
>> +preallocate memory or read the underlying file; files with holes might or
>> +might not preallocate blocks.
>> +Do not generate
>> +.B SIGBUS
>> +when populating fails, return an error instead.
>> +.IP
>> +If
>> +.B MADV_POPULATE_READ
>> +succeeds, all page tables have been populated (prefaulted) readable once.
>> +If
>> +.B MADV_POPULATE_READ
>> +fails, some page tables might have been populated.
>> +.IP
>> +.B MADV_POPULATE_READ
>> +cannot be applied to mappings without read permissions
>> +and special mappings marked with the kernel-internal
>> +.B VM_PFNMAP
>> +and
>> +.BR VM_IO .
>> +.IP
>> +Note that with
>> +.BR MADV_POPULATE_READ ,
>> +the process can be killed at any moment when the system runs out of memory.
>> +.TP
>> +.BR MADV_POPULATE_WRITE " (since Linux 5.14)
>> +Populate (prefault) page tables writable for the whole range without actually
>> +writing. Depending on the underlying mapping, preallocate memory or read the
> 
> Is this read or write?
> just reading and trying to understand :)

It's reading. Assume you have a file with existing content mapped into a 
process. Once you touch a page (read/write/execute) that maps to blocks 
with existing content, you'll have to load these blocks from disk first.

Thanks! :)

-- 
Thanks,

David / dhildenb