Re: [v2 PATCH 1/2] mm: mempolicy: make the behavior consistent when MPOL_MF_MOVE* and MPOL_MF_STRICT were specified

Vlastimil Babka
2019-07-17T10:56:03Z

On 7/16/19 7:18 PM, Yang Shi wrote:
>> I think after your patch, you miss putback_movable_pages() in cases
>> where some were queued, and later the walk returned -EIO. The previous
>> code doesn't miss it, but it's also not obvious due to the multiple if
>> (!err) checks. I would rewrite it some thing like this:
>>
>> if (ret < 0) {
>> putback_movable_pages(&pagelist);
>> err = ret;
>> goto mmap_out; // a new label above up_write()
>> }
>
> Yes, the old code had putback_movable_pages called when !err. But, I
> think that is for error handling of mbind_range() if I understand it
> correctly since if queue_pages_range() returns -EIO (only MPOL_MF_STRICT
> was specified and there was misplaced page) that page list should be
> empty . The old code should checked whether that list is empty or not.

Hm I guess you're right, returning with EIO means nothing was queued.
> So, in the new code I just removed that.
>
>>
>> The rest can have reduced identation now.
>
> Yes, the goto does eliminate the extra indentation.
>
>>
>>> + else {
>>> + err = mbind_range(mm, start, end, new);
>>>
>>> - if (nr_failed && (flags & MPOL_MF_STRICT))
>>> - err = -EIO;
>>> - } else
>>> - putback_movable_pages(&pagelist);
>>> + if (!err) {
>>> + int nr_failed = 0;
>>> +
>>> + if (!list_empty(&pagelist)) {
>>> + WARN_ON_ONCE(flags & MPOL_MF_LAZY);
>>> + nr_failed = migrate_pages(&pagelist, new_page,
>>> + NULL, start, MIGRATE_SYNC,
>>> + MR_MEMPOLICY_MBIND);
>>> + if (nr_failed)
>>> + putback_movable_pages(&pagelist);
>>> + }
>>> +
>>> + if ((ret > 0) ||
>>> + (nr_failed && (flags & MPOL_MF_STRICT)))
>>> + err = -EIO;
>>> + } else
>>> + putback_movable_pages(&pagelist);
>> While at it, IIRC the kernel style says that when the 'if' part uses
>> '{ }' then the 'else' part should as well, and it shouldn't be mixed.
>
> Really? The old code doesn't have '{ }' for else, and checkpatch doesn't
> report any error or warning.

Checkpatch probably doesn't catch it, nor did the reviewers of the older
code. But coding-style.rst says:

Do not unnecessarily use braces where a single statement will do.

...

This does not apply if only one branch of a conditional statement is a
single
statement; in the latter case use braces in both branches:

.. code-block:: c

if (condition) {
do_this();
do_that();
} else {
otherwise();
}

Thanks,
Vlastimil