# Re: \[PATCH v1\] madvise.2: Document MADV\_POPULATE\_READ and MADV\_POPULATE\_WRITE

- From: David Hildenbrand \<hidden\>
- Date: 2021-07-12 10:04:06 
- List: linux-mm
- Page: https://ratatoskr.run/linux-mm/2021/07/6509757
- Thread: https://ratatoskr.run/linux-man/2021/07/6509755/t.md

```text
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 <redacted>
>> Cc: Michael Kerrisk <redacted>
>> 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 <redacted>
>> Cc: <redacted>
>> Signed-off-by: David Hildenbrand <redacted>
>> ---
>>   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


```