Messages in this thread | | | Subject | Re: [PATCH 09/15] mm/swap: avoid calling swp_swap_info when try to check SWP_STABLE_WRITES | From | Miaohe Lin <> | Date | Tue, 10 May 2022 10:21:21 +0800 |
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On 2022/5/10 8:28, NeilBrown wrote: > On Mon, 09 May 2022, Miaohe Lin wrote: >> Use flags of si directly to check SWP_STABLE_WRITES to avoid possible >> READ_ONCE and thus save some cpu cycles. >> >> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> >> --- >> mm/memory.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 9c3e7e6ac202..89dd15504f3d 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -3892,7 +3892,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> */ >> exclusive = true; >> } else if (exclusive && PageWriteback(page) && >> - (swp_swap_info(entry)->flags & SWP_STABLE_WRITES)) { >> + (si->flags & SWP_STABLE_WRITES)) { > > Should this have a data_race() annotation. Other bit-tests of si->flags > do because scan_swap_map_slots can update it asynchronously, but we know > that won't matter in practice.
Yes, you're right. scan_swap_map_slots can update si->flags asynchronously while do_swap_page tests SWP_STABLE_WRITES here. We know this is harmless because SWP_STABLE_WRITES is only changed at swapon/swapoff.
Will add data_race() annotation in next version to avoid possible KCSAN data-race complaint.
Many thanks for pointing this out! :)
> > Thanks, > NeilBrown > > >> /* >> * This is tricky: not all swap backends support >> * concurrent page modifications while under writeback. >> -- >> 2.23.0 >> >> > . >
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