Re: [v2 PATCH] mm: move_pages: report the number of non-attempted pages

[Wei Yang]

On Fri, Jan 24, 2020 at 09:48:30AM -0800, Yang Shi wrote:
>
>
>On 1/24/20 7:26 AM, Wei Yang wrote:
>> On Fri, Jan 24, 2020 at 07:46:49AM +0100, Michal Hocko wrote:
>> > On Fri 24-01-20 06:56:47, Wei Yang wrote:
>> > > On Thu, Jan 23, 2020 at 09:55:26AM +0100, Michal Hocko wrote:
>> > > > On Thu 23-01-20 11:27:36, Wei Yang wrote:
>> > > > > On Thu, Jan 23, 2020 at 07:38:51AM +0800, Yang Shi wrote:
>> > > > > > Since commit a49bd4d71637 ("mm, numa: rework do_pages_move"),
>> > > > > > the semantic of move_pages() was changed to return the number of
>> > > > > > non-migrated pages (failed to migration) and the call would be aborted
>> > > > > > immediately if migrate_pages() returns positive value.  But it didn't
>> > > > > > report the number of pages that we even haven't attempted to migrate.
>> > > > > > So, fix it by including non-attempted pages in the return value.
>> > > > > > 
>> > > > > First, we want to change the semantic of move_pages(2). The return value
>> > > > > indicates the number of pages we didn't managed to migrate?
>> > > > > 
>> > > > > Second, the return value from migrate_pages() doesn't mean the number of pages
>> > > > > we failed to migrate. For example, one -ENOMEM is returned on the first page,
>> > > > > migrate_pages() would return 1. But actually, no page successfully migrated.
>> > > > ENOMEM is considered a permanent failure and as such it is returned by
>> > > > migrate pages (see goto out).
>> > > > 
>> > > > > Third, even the migrate_pages() return the exact non-migrate page, we are not
>> > > > > sure those non-migrated pages are at the tail of the list. Because in the last
>> > > > > case in migrate_pages(), it just remove the page from list. It could be a page
>> > > > > in the middle of the list. Then, in userspace, how the return value be
>> > > > > leveraged to determine the valid status? Any page in the list could be the
>> > > > > victim.
>> > > > Yes, I was wrong when stating that the caller would know better which
>> > > > status to check. I misremembered the original patch as it was quite some
>> > > > time ago. While storing the error code would be possible after some
>> > > > massaging of migrate_pages is this really something we deeply care
>> > > > about. The caller can achieve the same by initializing the status array
>> > > > to a non-node number - e.g. -1 - and check based on that.
>> > > > 
>> > > So for a user, the best practice is to initialize the status array to -1 and
>> > > check each status to see whether the page is migrated successfully?
>> > Yes IMO. Just consider -errno return value. You have no way to find out
>> > which pages have been migrated until we reached that error. The
>> > possitive return value would fall into the same case.
>> > 
>> > > Then do we need to return the number of non-migrated page? What benefit could
>> > > user get from the number. How about just return an error code to indicate the
>> > > failure? I may miss some point, would you mind giving me a hint?
>> > This is certainly possible. We can return -EAGAIN if some pages couldn't
>> > be migrated because they are pinned. But please read my previous email
>> > to the very end for arguments why this might cause more problems than it
>> > actually solves.
>> > 
>> Let me put your comment here:
>> 
>>      Because new users could have started depending on it. It
>>      is not all that unlikely that the current implementation would just
>>      work for them because they are migrating a set of pages on to the same
>>      node so the batch would be a single list throughout the whole given
>>      page set.
>> 
>> Your idea is to preserve current semantic, return non-migrated pages number to
>> userspace.
>> 
>> And the reason is:
>> 
>>     1. Users have started depending on it.
>>     2. No real bug reported yet.
>>     3. User always migrate page to the same node. (If my understanding is
>>        correct)
>> 
>> I think this gets some reason, since we want to minimize the impact to
>> userland.
>> 
>> While let's see what user probably use this syscall. Since from the man page,
>> we never told the return value could be positive, the number of non-migrated
>> pages, user would think only 0 means a successful migration and all other
>> cases are failure. Then user probably handle negative and positive return
>> value the same way, like (!err).
>> 
>> If my guess is true, return a negative error value for this case could
>> minimize the impact to userland here.
>>     1. Preserve the semantic of move_pages(2): 0 means success, negative means
>>        some error and needs extra handling.
>>     2. Trivial change to the man page.
>>     3. Suppose no change to users.
>> 
>> Well, in case I missed your point, sorry about that.
>
>I think we should compare the new semantic with the old one. With the old
>semantic the move_pages() return 0 for both success *and* migration failure.
>So, I'm supposed (I don't have any real usecase) the user may do the below
>with the old semantic:
>    - Just check if it is failed (ignore migration failure), "!err" is good
>enough.  This usecase is fine as well with the new semantic since migration
>failure is also a kind of error cases.
>     - Care about migration failure, the user needs traverse all bits in the
>status array. With the new semantic they just need check if "err > 0", if
>they want to know what specific pages are failed to migrate, then traverse
>the status array (with initialized as -1 as Michal suggested in earlier
>email).
>
>So, with returning errno for migration failure if the userspace wants to see
>if migration is failed, they need do:
>    1. Check "!err"
>    2. Read errno if #1 returns false
>    3. Traverse status array to see how many pages are failed to migrate
>

You are right. I misunderstand the mechanism of error handling on err and
errno.

>But with the new semantic they just need check if "err > 0", one step is fine
>for the most cases. So I said this approach seems more straightforward to the
>userspace and makes more sense IMHO.
>
>> > > > This system call has quite a complex semantic and I am not 100% sure
>> > > > what is the right thing to do here. Maybe we do want to continue and try
>> > > > to migrate as much as possible on non-fatal migration failures and
>> > > > accumulate the number of failed pages while doing so.
>> > > > 
>> > > > The main problem is that we can have an academic discussion but
>> > > > the primary question is what do actual users want. A lack of real
>> > > > bug reports suggests that nobody has actually noticed this. So I
>> > > > would rather keep returning the correct number of non-migrated
>> > > > pages. Why? Because new users could have started depending on it. It
>> > > > is not all that unlikely that the current implementation would just
>> > > > work for them because they are migrating a set of pages on to the same
>> > > > node so the batch would be a single list throughout the whole given
>> > > > page set.
>> > -- 
>> > Michal Hocko
>> > SUSE Labs

-- 
Wei Yang
Help you, Help me