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.bulw...@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