* Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()
@ 2020-10-05 6:58 ` Lukas Bulwahn
0 siblings, 0 replies; 11+ messages in thread
From: Lukas Bulwahn @ 2020-10-05 6:58 UTC (permalink / raw)
To: Mel Gorman
Cc: Lukas Bulwahn, Andrew Morton, linux-mm, Vlastimil Babka,
Michal Hocko, Nathan Chancellor, Nick Desaulniers, linux-kernel,
clang-built-linux, kernel-janitors, linux-safety
On Sun, 4 Oct 2020, Mel Gorman wrote:
> On Sun, Oct 04, 2020 at 02:58:27PM +0200, Lukas Bulwahn wrote:
> > The refactoring to kswapd() in commit e716f2eb24de ("mm, vmscan: prevent
> > kswapd sleeping prematurely due to mismatched classzone_idx") turned an
> > assignment to reclaim_order into a dead store, as in all further paths,
> > reclaim_order will be assigned again before it is used.
> >
> > make clang-analyzer on x86_64 tinyconfig caught my attention with:
> >
> > mm/vmscan.c: warning: Although the value stored to 'reclaim_order' is
> > used in the enclosing expression, the value is never actually read from
> > 'reclaim_order' [clang-analyzer-deadcode.DeadStores]
> >
> > Compilers will detect this unneeded assignment and optimize this anyway.
> > So, the resulting binary is identical before and after this change.
> >
> > Simplify the code and remove unneeded assignment to make clang-analyzer
> > happy.
> >
> > No functional change. No change in binary code.
> >
> > Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
>
> I'm not really keen on this. With the patch, reclaim_order can be passed
> uninitialised to kswapd_try_to_sleep. While a sufficiently smart
> compiler might be able to optimise how reclaim_order is used, it's not
> guaranteed either. Similarly, a change in kswapd_try_to_sleep and its
> called functions could rely on reclaim_order being a valid value and
> then introduce a subtle bug.
>
Just for my own understanding:
How would you see reclaim_order being passed unitialised to
kswapd_try_to_sleep?
From kswapd() entry, any path must reach the line
alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);
before kswap_try_to_sleep(...).
Then it reads back the order into alloc_order and reclaim_order
and resets pgdat->kswapd to 0.
I argue that the second store to reclaim_order is not used.
Path kthread_should_stop() is true:
Then, it either exits and does not use those temporary values,
reclaim_order and alloc_order, at all.
Path try_to_freeze() is true:
It goes back to the beginning of the loop and repeats reading alloc_order
and reclaim_order after the reset to 0, and then passes that to
kswapd_try_to_sleep(...). Previous reclaim_order is not used.
So, the previous store to alloc_order and reclaim_order is lost.
(Is that intentional?)
Path try_to_freeze() is false:
We call trace_mm_vmscan_kswapd_wake with alloc_order but not with
reclaim_order. reclaim_order is set by the return of balance_pgdat(...);
So, the previous reclaim_order is again not used.
The diff in the patch might be a bit small, but we are looking at the
second assignment after kswapd_try_to_sleep(...), not the first assignment
that just looks the same.
Lukas
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()
@ 2020-10-05 6:58 ` Lukas Bulwahn
0 siblings, 0 replies; 11+ messages in thread
From: Lukas Bulwahn @ 2020-10-05 6:58 UTC (permalink / raw)
To: Mel Gorman
Cc: Lukas Bulwahn, Andrew Morton, linux-mm, Vlastimil Babka,
Michal Hocko, Nathan Chancellor, Nick Desaulniers, linux-kernel,
clang-built-linux, kernel-janitors, linux-safety
On Sun, 4 Oct 2020, Mel Gorman wrote:
> On Sun, Oct 04, 2020 at 02:58:27PM +0200, Lukas Bulwahn wrote:
> > The refactoring to kswapd() in commit e716f2eb24de ("mm, vmscan: prevent
> > kswapd sleeping prematurely due to mismatched classzone_idx") turned an
> > assignment to reclaim_order into a dead store, as in all further paths,
> > reclaim_order will be assigned again before it is used.
> >
> > make clang-analyzer on x86_64 tinyconfig caught my attention with:
> >
> > mm/vmscan.c: warning: Although the value stored to 'reclaim_order' is
> > used in the enclosing expression, the value is never actually read from
> > 'reclaim_order' [clang-analyzer-deadcode.DeadStores]
> >
> > Compilers will detect this unneeded assignment and optimize this anyway.
> > So, the resulting binary is identical before and after this change.
> >
> > Simplify the code and remove unneeded assignment to make clang-analyzer
> > happy.
> >
> > No functional change. No change in binary code.
> >
> > Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
>
> I'm not really keen on this. With the patch, reclaim_order can be passed
> uninitialised to kswapd_try_to_sleep. While a sufficiently smart
> compiler might be able to optimise how reclaim_order is used, it's not
> guaranteed either. Similarly, a change in kswapd_try_to_sleep and its
> called functions could rely on reclaim_order being a valid value and
> then introduce a subtle bug.
>
Just for my own understanding:
How would you see reclaim_order being passed unitialised to
kswapd_try_to_sleep?
From kswapd() entry, any path must reach the line
alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);
before kswap_try_to_sleep(...).
Then it reads back the order into alloc_order and reclaim_order
and resets pgdat->kswapd to 0.
I argue that the second store to reclaim_order is not used.
Path kthread_should_stop() is true:
Then, it either exits and does not use those temporary values,
reclaim_order and alloc_order, at all.
Path try_to_freeze() is true:
It goes back to the beginning of the loop and repeats reading alloc_order
and reclaim_order after the reset to 0, and then passes that to
kswapd_try_to_sleep(...). Previous reclaim_order is not used.
So, the previous store to alloc_order and reclaim_order is lost.
(Is that intentional?)
Path try_to_freeze() is false:
We call trace_mm_vmscan_kswapd_wake with alloc_order but not with
reclaim_order. reclaim_order is set by the return of balance_pgdat(...);
So, the previous reclaim_order is again not used.
The diff in the patch might be a bit small, but we are looking at the
second assignment after kswapd_try_to_sleep(...), not the first assignment
that just looks the same.
Lukas
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()
2020-10-05 6:58 ` Lukas Bulwahn
@ 2020-10-05 7:56 ` Mel Gorman
-1 siblings, 0 replies; 11+ messages in thread
From: Mel Gorman @ 2020-10-05 7:56 UTC (permalink / raw)
To: Lukas Bulwahn
Cc: Andrew Morton, linux-mm, Vlastimil Babka, Michal Hocko,
Nathan Chancellor, Nick Desaulniers, linux-kernel,
clang-built-linux, kernel-janitors, linux-safety
On Mon, Oct 05, 2020 at 08:58:53AM +0200, Lukas Bulwahn wrote:
>
>
> On Sun, 4 Oct 2020, Mel Gorman wrote:
>
> > On Sun, Oct 04, 2020 at 02:58:27PM +0200, Lukas Bulwahn wrote:
> > > The refactoring to kswapd() in commit e716f2eb24de ("mm, vmscan: prevent
> > > kswapd sleeping prematurely due to mismatched classzone_idx") turned an
> > > assignment to reclaim_order into a dead store, as in all further paths,
> > > reclaim_order will be assigned again before it is used.
> > >
> > > make clang-analyzer on x86_64 tinyconfig caught my attention with:
> > >
> > > mm/vmscan.c: warning: Although the value stored to 'reclaim_order' is
> > > used in the enclosing expression, the value is never actually read from
> > > 'reclaim_order' [clang-analyzer-deadcode.DeadStores]
> > >
> > > Compilers will detect this unneeded assignment and optimize this anyway.
> > > So, the resulting binary is identical before and after this change.
> > >
> > > Simplify the code and remove unneeded assignment to make clang-analyzer
> > > happy.
> > >
> > > No functional change. No change in binary code.
> > >
> > > Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
> >
> > I'm not really keen on this. With the patch, reclaim_order can be passed
> > uninitialised to kswapd_try_to_sleep. While a sufficiently smart
> > compiler might be able to optimise how reclaim_order is used, it's not
> > guaranteed either. Similarly, a change in kswapd_try_to_sleep and its
> > called functions could rely on reclaim_order being a valid value and
> > then introduce a subtle bug.
> >
>
> Just for my own understanding:
>
> How would you see reclaim_order being passed unitialised to
> kswapd_try_to_sleep?
>
> From kswapd() entry, any path must reach the line
>
> alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);
>
> before kswap_try_to_sleep(...).
>
After your patch, the code is
unsigned int alloc_order, reclaim_order;
...
for ( ; ; ) {
alloc_order = READ_ONCE(pgdat->kswapd_order);
highest_zoneidx = kswapd_highest_zoneidx(pgdat,
highest_zoneidx);
kswapd_try_sleep:
kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,
highest_zoneidx);
...
reclaim_order = balance_pgdat(pgdat, alloc_order,
highest_zoneidx);
reclaim_order is declared, not initialised at the start of the loop and
passed into kswapd_try_to_sleep. There is a sequence where it is not used
so it does not matter but it depends on the compiler figuring that out.
--
Mel Gorman
SUSE Labs
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()
@ 2020-10-05 7:56 ` Mel Gorman
0 siblings, 0 replies; 11+ messages in thread
From: Mel Gorman @ 2020-10-05 7:56 UTC (permalink / raw)
To: Lukas Bulwahn
Cc: Andrew Morton, linux-mm, Vlastimil Babka, Michal Hocko,
Nathan Chancellor, Nick Desaulniers, linux-kernel,
clang-built-linux, kernel-janitors, linux-safety
On Mon, Oct 05, 2020 at 08:58:53AM +0200, Lukas Bulwahn wrote:
>
>
> On Sun, 4 Oct 2020, Mel Gorman wrote:
>
> > On Sun, Oct 04, 2020 at 02:58:27PM +0200, Lukas Bulwahn wrote:
> > > The refactoring to kswapd() in commit e716f2eb24de ("mm, vmscan: prevent
> > > kswapd sleeping prematurely due to mismatched classzone_idx") turned an
> > > assignment to reclaim_order into a dead store, as in all further paths,
> > > reclaim_order will be assigned again before it is used.
> > >
> > > make clang-analyzer on x86_64 tinyconfig caught my attention with:
> > >
> > > mm/vmscan.c: warning: Although the value stored to 'reclaim_order' is
> > > used in the enclosing expression, the value is never actually read from
> > > 'reclaim_order' [clang-analyzer-deadcode.DeadStores]
> > >
> > > Compilers will detect this unneeded assignment and optimize this anyway.
> > > So, the resulting binary is identical before and after this change.
> > >
> > > Simplify the code and remove unneeded assignment to make clang-analyzer
> > > happy.
> > >
> > > No functional change. No change in binary code.
> > >
> > > Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
> >
> > I'm not really keen on this. With the patch, reclaim_order can be passed
> > uninitialised to kswapd_try_to_sleep. While a sufficiently smart
> > compiler might be able to optimise how reclaim_order is used, it's not
> > guaranteed either. Similarly, a change in kswapd_try_to_sleep and its
> > called functions could rely on reclaim_order being a valid value and
> > then introduce a subtle bug.
> >
>
> Just for my own understanding:
>
> How would you see reclaim_order being passed unitialised to
> kswapd_try_to_sleep?
>
> From kswapd() entry, any path must reach the line
>
> alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);
>
> before kswap_try_to_sleep(...).
>
After your patch, the code is
unsigned int alloc_order, reclaim_order;
...
for ( ; ; ) {
alloc_order = READ_ONCE(pgdat->kswapd_order);
highest_zoneidx = kswapd_highest_zoneidx(pgdat,
highest_zoneidx);
kswapd_try_sleep:
kswapd_try_to_sleep(pgdat, alloc_order, reclaim_order,
highest_zoneidx);
...
reclaim_order = balance_pgdat(pgdat, alloc_order,
highest_zoneidx);
reclaim_order is declared, not initialised at the start of the loop and
passed into kswapd_try_to_sleep. There is a sequence where it is not used
so it does not matter but it depends on the compiler figuring that out.
--
Mel Gorman
SUSE Labs
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()
2020-10-05 6:58 ` Lukas Bulwahn
@ 2020-10-05 7:59 ` Mel Gorman
-1 siblings, 0 replies; 11+ messages in thread
From: Mel Gorman @ 2020-10-05 7:59 UTC (permalink / raw)
To: Lukas Bulwahn
Cc: Andrew Morton, linux-mm, Vlastimil Babka, Michal Hocko,
Nathan Chancellor, Nick Desaulniers, linux-kernel,
clang-built-linux, kernel-janitors, linux-safety
On Mon, Oct 05, 2020 at 08:58:53AM +0200, Lukas Bulwahn wrote:
>
>
> On Sun, 4 Oct 2020, Mel Gorman wrote:
>
> > On Sun, Oct 04, 2020 at 02:58:27PM +0200, Lukas Bulwahn wrote:
> > > The refactoring to kswapd() in commit e716f2eb24de ("mm, vmscan: prevent
> > > kswapd sleeping prematurely due to mismatched classzone_idx") turned an
> > > assignment to reclaim_order into a dead store, as in all further paths,
> > > reclaim_order will be assigned again before it is used.
> > >
> > > make clang-analyzer on x86_64 tinyconfig caught my attention with:
> > >
> > > mm/vmscan.c: warning: Although the value stored to 'reclaim_order' is
> > > used in the enclosing expression, the value is never actually read from
> > > 'reclaim_order' [clang-analyzer-deadcode.DeadStores]
> > >
> > > Compilers will detect this unneeded assignment and optimize this anyway.
> > > So, the resulting binary is identical before and after this change.
> > >
> > > Simplify the code and remove unneeded assignment to make clang-analyzer
> > > happy.
> > >
> > > No functional change. No change in binary code.
> > >
> > > Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
> >
> > I'm not really keen on this. With the patch, reclaim_order can be passed
> > uninitialised to kswapd_try_to_sleep. While a sufficiently smart
> > compiler might be able to optimise how reclaim_order is used, it's not
> > guaranteed either. Similarly, a change in kswapd_try_to_sleep and its
> > called functions could rely on reclaim_order being a valid value and
> > then introduce a subtle bug.
> >
>
> Just for my own understanding:
>
> How would you see reclaim_order being passed unitialised to
> kswapd_try_to_sleep?
>
> From kswapd() entry, any path must reach the line
>
> alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);
>
> before kswap_try_to_sleep(...).
>
Bah, I misread the patch because I'm an idiot.
Acked-by: Mel Gorman <mgorman@techsingularity.net>
--
Mel Gorman
SUSE Labs
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()
@ 2020-10-05 7:59 ` Mel Gorman
0 siblings, 0 replies; 11+ messages in thread
From: Mel Gorman @ 2020-10-05 7:59 UTC (permalink / raw)
To: Lukas Bulwahn
Cc: Andrew Morton, linux-mm, Vlastimil Babka, Michal Hocko,
Nathan Chancellor, Nick Desaulniers, linux-kernel,
clang-built-linux, kernel-janitors, linux-safety
On Mon, Oct 05, 2020 at 08:58:53AM +0200, Lukas Bulwahn wrote:
>
>
> On Sun, 4 Oct 2020, Mel Gorman wrote:
>
> > On Sun, Oct 04, 2020 at 02:58:27PM +0200, Lukas Bulwahn wrote:
> > > The refactoring to kswapd() in commit e716f2eb24de ("mm, vmscan: prevent
> > > kswapd sleeping prematurely due to mismatched classzone_idx") turned an
> > > assignment to reclaim_order into a dead store, as in all further paths,
> > > reclaim_order will be assigned again before it is used.
> > >
> > > make clang-analyzer on x86_64 tinyconfig caught my attention with:
> > >
> > > mm/vmscan.c: warning: Although the value stored to 'reclaim_order' is
> > > used in the enclosing expression, the value is never actually read from
> > > 'reclaim_order' [clang-analyzer-deadcode.DeadStores]
> > >
> > > Compilers will detect this unneeded assignment and optimize this anyway.
> > > So, the resulting binary is identical before and after this change.
> > >
> > > Simplify the code and remove unneeded assignment to make clang-analyzer
> > > happy.
> > >
> > > No functional change. No change in binary code.
> > >
> > > Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
> >
> > I'm not really keen on this. With the patch, reclaim_order can be passed
> > uninitialised to kswapd_try_to_sleep. While a sufficiently smart
> > compiler might be able to optimise how reclaim_order is used, it's not
> > guaranteed either. Similarly, a change in kswapd_try_to_sleep and its
> > called functions could rely on reclaim_order being a valid value and
> > then introduce a subtle bug.
> >
>
> Just for my own understanding:
>
> How would you see reclaim_order being passed unitialised to
> kswapd_try_to_sleep?
>
> From kswapd() entry, any path must reach the line
>
> alloc_order = reclaim_order = READ_ONCE(pgdat->kswapd_order);
>
> before kswap_try_to_sleep(...).
>
Bah, I misread the patch because I'm an idiot.
Acked-by: Mel Gorman <mgorman@techsingularity.net>
--
Mel Gorman
SUSE Labs
^ permalink raw reply [flat|nested] 11+ messages in thread