On 3/21/19 12:45 PM, Michal Hocko wrote:
On Thu 21-03-19 10:25:08, Yang Shi wrote:
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 have overlook that MPOL_MF_VALID doesn't include MPOL_MF_LAZY. Anyway,
my argument still holds that the bit has to be reserved for ever because
it used to be valid at some point of time and not returning EINVAL could
imply you are running on the kernel which supports the flag.
I'd say it is not valid since very beginning. MPOL_MF_LAZY was added by
commit b24f53a0bea3 ("mm: mempolicy: Add
MPOL_MF_LAZY"), then it was hidden by commit a720094ded8c ("mm:
mempolicy: Hide MPOL_NOOP and MPOL_MF_LAZY from userspace for now").
And, git describe --contains shows:
US-143344MP:linux yang.s$ git describe --contains b24f53a0bea3
v3.8-rc1~92^2~27
US-143344MP:linux yang.s$ git describe --contains a720094ded8c
v3.8-rc1~92^2~25
This is why I thought it is never ever exported to userspace.
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.
As Mel already pointed out. This doesn't really sound like a sound
argument. Say we would remove those few lines of code and preserve the
flag for future reservation of the flag bit. I would bet my head that it
will not be long before somebody just goes and clean it up and remove
because the flag is unused. So you would have to put a note explaining
why this has to be preserved. Maybe the current code is better to
document that. It would be much more sound to remove the code if it was
causing a measurable overhead or a maintenance burden. Is any of that
the case?
As what I found out, I just thought it may be dead code, if so why not
remove it otherwise we may have to keep maintaining the unused code.
Thanks,
Yang
