On Mon, Dec 17, 2018 at 07:51:27PM +0900, Tetsuo Handa wrote:
> On 2018/12/17 18:33, Michal Hocko wrote:
> > On Sun 16-12-18 19:51:57, Matthew Wilcox wrote:
> > [...]
> >> Ah, yes, that makes perfect sense. Thank you for the explanation.
> >>
> >> I wonder if the correct fix, however, is not to move the check for
> >> GFP_NOFS in out_of_memory() down to below the check whether to kill
> >> the current task. That would solve your problem, and I don't _think_
> >> it would cause any new ones. Michal, you touched this code last, what
> >> do you think?
> >
> > What do you mean exactly? Whether we kill a current task or something
> > else doesn't change much on the fact that NOFS is a reclaim restricted
> > context and we might kill too early. If the fs can do GFP_FS then it is
> > obviously a better thing to do because FS metadata can be reclaimed as
> > well and therefore there is potentially less memory pressure on
> > application data.
> >
>
> I interpreted "to move the check for GFP_NOFS in out_of_memory() down to
> below the check whether to kill the current task" as
Too far; I meant one line earlier, before we try to select a different
process.
> @@ -1104,6 +1095,19 @@ bool out_of_memory(struct oom_control *oc)
> }
>
> select_bad_process(oc);
> +
> + /*
> + * The OOM killer does not compensate for IO-less reclaim.
> + * pagefault_out_of_memory lost its gfp context so we have to
> + * make sure exclude 0 mask - all other users should have at least
> + * ___GFP_DIRECT_RECLAIM to get here.
> + */
> + if ((oc->gfp_mask && !(oc->gfp_mask & __GFP_FS)) && oc->chosen &&
> + oc->chosen != (void *)-1UL && oc->chosen != current) {
> + put_task_struct(oc->chosen);
> + return true;
> + }
> +
> /* Found nothing?!?! */
> if (!oc->chosen) {
> dump_header(oc, NULL);
>
> which is prefixed by "the correct fix is not".
>
> Behaving like sysctl_oom_kill_allocating_task == 1 if __GFP_FS is not used
> will not be the correct fix. But ...
>
> Hou Tao wrote:
> > There is no need to disable __GFP_FS in ->readpage:
> > * It's a read-only fs, so there will be no dirty/writeback page and
> > there will be no deadlock against the caller's locked page
>
> is read-only filesystem sufficient for safe to use __GFP_FS?
>
> Isn't "whether it is safe to use __GFP_FS" depends on "whether fs locks
> are held or not" rather than "whether fs has dirty/writeback page or not" ?
It's worth noticing that squashfs _is_ in fact holding a page locked in
squashfs_copy_cache() when it calls grab_cache_page_nowait(). I'm not
sure if this will lead to trouble or not because I'm insufficiently
familiar with the reclaim path.
