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

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

Hi David, Pankaj,

On 7/12/21 11:58 AM, 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.

Please see a couple of comments below.

>>
>> 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>
 > Acked-by: Pankaj Gupta <pankaj.gupta@ionos.com>

Thanks for the acked by!

Cheers,

Alex

>> ---
>>   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)

s/$/"/

>> +Populate (prefault) page tables readable for the whole range without actually

See the following extract from man-pages(7):

$ man 7 man-pages | sed -n '/Use semantic newlines/,/^$/p';
    Use semantic newlines
        In the source of a manual page,  new  sentences  should  be
        started  on new lines, and long sentences should split into
        lines at clause breaks (commas, semicolons, colons, and  so
        on).   This  convention,  sometimes known as "semantic new‐
        lines", makes it easier to see the effect of patches, which
        often  operate at the level of individual sentences or sen‐
        tence clauses.

>> +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)

s/$/"/

>> +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 :)
> 
>> +underlying file; files with holes will preallocate blocks.
>> +Do not generate
>> +.B SIGBUS
>> +when populating fails, return an error instead.
>> +.IP
>> +If
>> +.B MADV_POPULATE_WRITE
>> +succeeds, all page tables have been populated (prefaulted) writable once.
>> +If
>> +.B MADV_POPULATE_WRITE
>> +fails, some page tables might have been populated.
>> +.IP
>> +.B MADV_POPULATE_WRITE
>> +cannot be applied to mappings without write permissions
>> +and special mappings marked with the kernel-internal
>> +.B VM_PFNMAP
>> +and
>> +.BR VM_IO .
>> +.IP
>> +Note that
>> +.BR MADV_POPULATE_WRITE ,
>> +the process can be killed at any moment when the system runs out of memory.
>>   .SH RETURN VALUE
>>   On success,
>>   .BR madvise ()
>> @@ -533,6 +586,17 @@ or
>>   .BR VM_PFNMAP
>>   ranges.
>>   .TP
>> +.B EINVAL
>> +.I advice
>> +is
>> +.B MADV_POPULATE_READ
>> +or
>> +.BR MADV_POPULATE_WRITE ,
>> +but the specified address range includes ranges with insufficient permissions,
>> +.B VM_IO
>> +or
>> +.BR VM_PFNMAP.
>> +.TP
>>   .B EIO
>>   (for
>>   .BR MADV_WILLNEED )
>> @@ -548,6 +612,14 @@ Not enough memory: paging in failed.
>>   Addresses in the specified range are not currently
>>   mapped, or are outside the address space of the process.
>>   .TP
>> +.B ENOMEM
>> +.I advice
>> +is
>> +.B MADV_POPULATE_READ
>> +or
>> +.BR MADV_POPULATE_WRITE ,
>> +but populating (prefaulting) page tables failed.
>> +.TP
>>   .B EPERM
>>   .I advice
>>   is
>> @@ -555,6 +627,14 @@ is
>>   but the caller does not have the
>>   .B CAP_SYS_ADMIN
>>   capability.
>> +.TP
>> +.B EHWPOISON
>> +.I advice
>> +is
>> +.B MADV_POPULATE_READ
>> +or
>> +.BR MADV_POPULATE_WRITE ,
>> +and a HW poisoned page is encountered.
>>   .SH VERSIONS
>>   Since Linux 3.18,
>>   .\" commit d3ac21cacc24790eb45d735769f35753f5b56ceb
>> --
>> 2.31.1
>>
>>


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/