Messages in this thread Patch in this message | | | Subject | Re: [RFC PATCH] mm: mempolicy: remove MPOL_MF_LAZY | From | Yang Shi <> | Date | Thu, 21 Mar 2019 10:25:08 -0700 |
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On 3/21/19 9:51 AM, Michal Hocko wrote: > 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.
Thanks, I see you point. Yes, it is exported to userspace in some sense since it is in uapi header. But, it is never documented and MPOL_MF_VALID excludes it. mbind() does check and mask it. It would return -EINVAL if MPOL_MF_LAZY or any other undefined/invalid flag is set. See the below code snippet from do_mbind():
... #define MPOL_MF_VALID (MPOL_MF_STRICT | \ MPOL_MF_MOVE | \ MPOL_MF_MOVE_ALL)
if (flags & ~(unsigned long)MPOL_MF_VALID) return -EINVAL;
So, I don't think any application would really use the flag for mbind() unless it is aimed to test the -EINVAL. If just test program, it should be not considered as a regression.
> >> 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.
As I elaborated above, mbind() syscall does check it and treat it as an invalid flag. MPOL_PREFERRED doesn't use it either, but just use MPOL_F_MOF directly.
Thanks, Yang
> >> Thanks, >> Yang >> >>>> Cc: Mel Gorman <mgorman@techsingularity.net> >>>> Cc: Michal Hocko <mhocko@suse.com> >>>> Cc: Vlastimil Babka <vbabka@suse.cz> >>>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> >>>> --- >>>> 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 >>>>
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