Re: [RFC PATCH] mm: mempolicy: remove MPOL_MF_LAZY

[Michal Hocko]

On Thu 21-03-19 09:21:39, Yang Shi wrote:
> 
> 
> On 3/21/19 7:57 AM, Michal Hocko wrote:
> > On Wed 20-03-19 08:27:39, Yang Shi wrote:
> > > MPOL_MF_LAZY was added by commit b24f53a0bea3 ("mm: mempolicy: Add
> > > MPOL_MF_LAZY"), then it was disabled by commit a720094ded8c ("mm:
> > > mempolicy: Hide MPOL_NOOP and MPOL_MF_LAZY from userspace for now")
> > > right away in 2012.  So, it is never ever exported to userspace.
> > > 
> > > And, it looks nobody is interested in revisiting it since it was
> > > disabled 7 years ago.  So, it sounds pointless to still keep it around.
> > The above changelog owes us a lot of explanation about why this is
> > safe and backward compatible. I am also not sure you can change
> > MPOL_MF_INTERNAL because somebody still might use the flag from
> > userspace and we want to guarantee it will have the exact same semantic.
> 
> Since MPOL_MF_LAZY is never exported to userspace (Mel helped to confirm
> this in the other thread), so I'm supposed it should be safe and backward
> compatible to userspace.

You didn't get my point. The flag is exported to the userspace and
nothing in the syscall entry path checks and masks it. So we really have
to preserve the semantic of the flag bit for ever.

> I'm also not sure if anyone use MPOL_MF_INTERNAL or not and how they use it
> in their applications, but how about keeping it unchanged?

You really have to. Because it is an offset of other MPLO flags for
internal usage.

That being said. Considering that we really have to preserve
MPOL_MF_LAZY value (we cannot even rename it because it is in uapi
headers and we do not want to break compilation). What is the point of
this change? Why is it an improvement? Yes, nobody is probably using
this because this is not respected in anything but the preferred mem
policy. At least that is the case from my quick glance. I might be still
wrong as it is quite easy to overlook all the consequences. So the risk
is non trivial while the benefit is not really clear to me. If you see
one, _document_ it. "Mel said it is not in use" is not a justification,
with all due respect.

> Thanks,
> Yang
> 
> > 
> > > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > > ---
> > > Hi folks,
> > > I'm not sure if you still would like to revisit it later. And, I may be
> > > not the first one to try to remvoe it. IMHO, it sounds pointless to still
> > > keep it around if nobody is interested in it.
> > > 
> > >   include/uapi/linux/mempolicy.h |  3 +--
> > >   mm/mempolicy.c                 | 13 -------------
> > >   2 files changed, 1 insertion(+), 15 deletions(-)
> > > 
> > > diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
> > > index 3354774..eb52a7a 100644
> > > --- a/include/uapi/linux/mempolicy.h
> > > +++ b/include/uapi/linux/mempolicy.h
> > > @@ -45,8 +45,7 @@ enum {
> > >   #define MPOL_MF_MOVE	 (1<<1)	/* Move pages owned by this process to conform
> > >   				   to policy */
> > >   #define MPOL_MF_MOVE_ALL (1<<2)	/* Move every page to conform to policy */
> > > -#define MPOL_MF_LAZY	 (1<<3)	/* Modifies '_MOVE:  lazy migrate on fault */
> > > -#define MPOL_MF_INTERNAL (1<<4)	/* Internal flags start here */
> > > +#define MPOL_MF_INTERNAL (1<<3)	/* Internal flags start here */
> > >   #define MPOL_MF_VALID	(MPOL_MF_STRICT   | 	\
> > >   			 MPOL_MF_MOVE     | 	\
> > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > > index af171cc..67886f4 100644
> > > --- a/mm/mempolicy.c
> > > +++ b/mm/mempolicy.c
> > > @@ -593,15 +593,6 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
> > >   	qp->prev = vma;
> > > -	if (flags & MPOL_MF_LAZY) {
> > > -		/* Similar to task_numa_work, skip inaccessible VMAs */
> > > -		if (!is_vm_hugetlb_page(vma) &&
> > > -			(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)) &&
> > > -			!(vma->vm_flags & VM_MIXEDMAP))
> > > -			change_prot_numa(vma, start, endvma);
> > > -		return 1;
> > > -	}
> > > -
> > >   	/* queue pages from current vma */
> > >   	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
> > >   		return 0;
> > > @@ -1181,9 +1172,6 @@ static long do_mbind(unsigned long start, unsigned long len,
> > >   	if (IS_ERR(new))
> > >   		return PTR_ERR(new);
> > > -	if (flags & MPOL_MF_LAZY)
> > > -		new->flags |= MPOL_F_MOF;
> > > -
> > >   	/*
> > >   	 * If we are using the default policy then operation
> > >   	 * on discontinuous address spaces is okay after all
> > > @@ -1226,7 +1214,6 @@ static long do_mbind(unsigned long start, unsigned long len,
> > >   		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)
> > > -- 
> > > 1.8.3.1
> > > 

-- 
Michal Hocko
SUSE Labs