Re: [PATCH] madvise.2: Clarify addr/length and update hugetlb support

[Mike Kravetz <mike.kravetz@oracle.com>]

On 5/25/22 11:38, Peter Xu wrote:
> Hi, Mike,
> 
> Some minor nitpicks below.
> 
> On Tue, May 24, 2022 at 04:28:44PM -0700, Mike Kravetz wrote:
>>  .B MADV_DONTNEED
>> -cannot be applied to locked pages, Huge TLB pages, or
>> +cannot be applied to locked pages, or
>>  .BR VM_PFNMAP
>>  pages.
> 
> This looks good, but since this will be a behavior change and we won't be
> able to change the old kernels, I saw the man page normally does this with
> things like:
> 
>   Since Linux 5.18, this madvise supports hugetlbfs page> 
> Majorly it states starting from which version it'll work, and when it'll
> not.

You are right.  I will add this.
> 
>>  (Pages marked with the kernel-internal
>> @@ -170,24 +174,24 @@ Note that some of these operations change the semantics of memory accesses.
>>  .\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349
>>  Free up a given range of pages
>>  and its associated backing store.
>> -This is equivalent to punching a hole in the corresponding byte
>> +This is equivalent to punching a hole in the corresponding
>>  range of the backing store (see
>>  .BR fallocate (2)).
>>  Subsequent accesses in the specified address range will see
>> -bytes containing zero.
>> +pages containing zero.
>>  .\" Databases want to use this feature to drop a section of their
>>  .\" bufferpool (shared memory segments) - without writing back to
>>  .\" disk/swap space.  This feature is also useful for supporting
>>  .\" hot-plug memory on UML.
>>  .IP
>>  The specified address range must be mapped shared and writable.
>> -This flag cannot be applied to locked pages, Huge TLB pages, or
>> +This flag cannot be applied to locked pages, or
>>  .BR VM_PFNMAP
>>  pages.
>>  .IP
>>  In the initial implementation, only
>>  .BR tmpfs (5)
>> -was supported
>> +supported
>>  .BR MADV_REMOVE ;
>>  but since Linux 3.5,
>>  .\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17
>> @@ -196,9 +200,9 @@ any filesystem which supports the
>>  .BR FALLOC_FL_PUNCH_HOLE
>>  mode also supports
>>  .BR MADV_REMOVE .
>> -Hugetlbfs fails with the error
>> -.BR EINVAL
>> -and other filesystems fail with the error
>> +Filesystems which do not support
>> +.BR MADV_REMOVE
>> +fail with the error
>>  .BR EOPNOTSUPP .
>>  .TP
>>  .BR MADV_DONTFORK " (since Linux 2.6.16)"
>> @@ -596,6 +600,18 @@ that are not mapped, the Linux version of
>>  ignores them and applies the call to the rest (but returns
>>  .B ENOMEM
>>  from the system call, as it should).
>> +.PP
>> +If the specified address
>> +.I addr
>> +is within a mapping backed by Huge TLB pages, then
>> +.I addr
>> +must be aligned to the underlying Huge TLB page size.  If the range
>> +specified by
>> +.I addr
>> +and
>> +.I length
>> +ends in a mapping backed by Huge TLB pages, then the end of the range
>> +will be rounded up to a multiple of the underlying Huge TLB page size.
> 
> I'm slightly worried this could be hidden too deep, meanwhile it duplicates
> part of the sentence of how start/end will be treated.

Yes, I just dumped more stuff into the NOTES section.  Will rearrange as
you suggested.

> 
> How about adding a short paragraph into each of MADV_DONTNEED and
> MADV_REMOVE section (right after the new sentences upon hugetlbfs), with:
> 
>   For hugetlbfs, the start/end alignments on page sizes will be based on
>   huge page size.
> 
> No strong opinions on any of these.  Anyway:
> 
> Acked-by: Peter Xu <peterx@redhat.com>

Thanks Peter
-- 
Mike Kravetz