Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes: > On Thu, May 28, 2020 at 01:32:40PM +0800, Huang, Ying wrote: >> Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes: >> >> > On Mon, May 25, 2020 at 08:26:48AM +0800, Huang Ying wrote: >> >> diff --git a/mm/swapfile.c b/mm/swapfile.c >> >> index 423c234aca15..0abd93d2a4fc 100644 >> >> --- a/mm/swapfile.c >> >> +++ b/mm/swapfile.c >> >> @@ -615,7 +615,8 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si, >> >> * discarding, do discard now and reclaim them >> >> */ >> >> swap_do_scheduled_discard(si); >> >> - *scan_base = *offset = si->cluster_next; >> >> + *scan_base = this_cpu_read(*si->cluster_next_cpu); >> >> + *offset = *scan_base; >> >> goto new_cluster; >> > >> > Why is this done? As far as I can tell, the values always get overwritten at >> > the end of the function with tmp and tmp isn't derived from them. Seems >> > ebc2a1a69111 moved some logic that used to make sense but doesn't have any >> > effect now. >> >> If we fail to allocate from cluster, "scan_base" and "offset" will not >> be overridden. > > Ok, if another task races to allocate the clusters the first just discarded. > >> And "cluster_next" or "cluster_next_cpu" may be changed >> in swap_do_scheduled_discard(), because the lock is released and >> re-acquired there. > > I see, by another task on the same cpu for cluster_next_cpu. > > Both probably unlikely, but at least it tries to pick up where the racing task > left off. You might tack this onto the comment: > > * discarding, do discard now and reclaim them, then reread > * cluster_next_cpu since we dropped si->lock > /* Sure. Will add this in the next version. >> The code may not have much value. > > No, it makes sense. > >> > These aside, patch looks good to me. >> >> Thanks for your review! It really help me to improve the quality of the >> patch. Can I add your "Reviewed-by" in the next version? > > Sure, > Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> Thanks! Best Regards, Huang, Ying
