Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

On 2019-10-31 at 12:08 Andrew Morton wrote:

(cc linux-man@vger.kernel.org)

On Tue, 29 Oct 2019 17:56:06 +0800 "Li Xinhai" [off-list ref] wrote:

quoted

queue_pages_range() will check for unmapped holes besides queue pages for
migration. The rules for checking unmapped holes are:
1 Unmapped holes at any part of the specified range should be reported as
  EFAULT if mbind() for none MPOL_DEFAULT cases;
2 Unmapped holes at any part of the specified range should be ignored if
  mbind() for MPOL_DEFAULT case;
Note that the second rule is the current implementation, but it seems
conflicts the Linux API definition.

Can you quote the part of the API definition which you're looking at?

My mbind(2) manpage says

ERRORS
      EFAULT Part or all of the memory range specified by nodemask and maxn-
             ode points outside your accessible address space.  Or, there was
             an unmapped hole in the specified memory range specified by addr
             and len.

(I assume the first sentence meant to say "specified by addr and len")

this part: 
"Or, there was an unmapped hole in the specified memory range specified by addr 
and len."
is concerned by my patch.

I agree with your interpretation, but there's no mention here that
MPOL_DEFAULT is treated differently and I don't see why it should be.

The first rule match the manpage, but the current mempolicy implementation only 
reports EFAULT if holes are within range, or at the head side of range. No EFAULT 
reported if hole at tail side of range. I suppose the first rule has to be fixed.

The seconde rule, when MPOL_DEFAULT is used, was summarized by me according 
to mempolicy implementation. Actually, this rule does not follow manpage and exsits 
for long days. In my understanding, this rule is reasonable (in code,  the internal flag 
MPOL_MF_DISCONTIG_OK is used for that purpose, there is comments for reason) 
and we'd better keep it.

More broadly, I worry that it's too late to change this - existing
applications might fail if we change the implementation in the proposed
fashion.  So perhaps what we should do here is to change the manpage to
match reality?

I prefer add description in manpage for the second rule, so no change to our code. 
Only fix for first rule.

Is the current behavior causing you any problems in a real-world use
case? 

I was using mbind() with MPOL_DEFAULT(or MPOL_BIND) to reset a range of address 
(which maybe contiguous or not in the whole range) to the default policy (to a specific 
node), and observed this issue. If mbind() call for each mapping one by one, we don't see the 
issue.

- Xinhai