linux-kernel - Re: [PATCH v21 14/19] mm/lru: introduce TestClearPageLRU

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Date:   Thu, 12 Nov 2020 12:24:17 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Alex Shi <alex.shi@...ux.alibaba.com>, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, tj@...nel.org,
        khlebnikov@...dex-team.ru, daniel.m.jordan@...cle.com,
        willy@...radead.org, hannes@...xchg.org, lkp@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, shakeelb@...gle.com,
        iamjoonsoo.kim@....com, richard.weiyang@...il.com,
        kirill@...temov.name, alexander.duyck@...il.com,
        rong.a.chen@...el.com, mhocko@...e.com, vdavydov.dev@...il.com,
        shy828301@...il.com, Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v21 14/19] mm/lru: introduce TestClearPageLRU

On 11/12/20 3:03 AM, Hugh Dickins wrote:
> On Wed, 11 Nov 2020, Vlastimil Babka wrote:
>> On 11/5/20 9:55 AM, Alex Shi wrote:
>> 
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -1542,7 +1542,7 @@ unsigned int reclaim_clean_pages_from_list(struct
>> > zone *zone,
>> >    */
>> >   int __isolate_lru_page(struct page *page, isolate_mode_t mode)
>> >   {
>> > -	int ret = -EINVAL;
>> > +	int ret = -EBUSY;
>> >     	/* Only take pages on the LRU. */
>> >   	if (!PageLRU(page))
>> > @@ -1552,8 +1552,6 @@ int __isolate_lru_page(struct page *page,
>> > isolate_mode_t mode)
>> >   	if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
>> >   		return ret;
>> >   -	ret = -EBUSY;
>> 
>> I'm not sure why this change is here, looks unrelated to the patch?
>> 
>> Oh I see, you want to prevent the BUG() in isolate_lru_pages().
> 
> Yes, I suggested this part of the patch to Alex, when I hit that BUG().
> 
>> 
>> But due to that, the PageUnevictable check was also affected unintentionally.
>> But I don't think it's that important to BUG() when we run into
>> PageUnevictable unexpectedly, so that's probably ok.
> 
> Not unintentional.  __isolate_lru_page(), or __isolate_lru_page_prepare(),
> is a silly function, used by two callers whose requirements are almost
> entirely disjoint.  The ISOLATE_UNEVICTABLE case is only for compaction.c,
> which takes no interest in -EINVAL versus -EBUSY, and has no such BUG().
> 
> I think it dates back to lumpy reclaim days, and it probably made more
> sense back then.

Ah, thanks for explaining.


>> 
>> But with that, we can just make __isolate_lru_page() a bool function and
>> remove the ugly switch in  isolate_lru_pages()?
> 
> I agree that the switch statement in isolate_lru_pages() seems pointless
> now, and can be turned into an if{}else{}.  But that cleanup is a
> diversion from this particular TestClearPageLRU patch, and I think from
> the whole series (checking final state of the patchset, yes, the switch
> is still there - though I think there have been variant series which
> removed it).
> 
> Can we please leave that cleanup until after the series has gone in?

Sure thing!

The patch seems functionally fine, so

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> I think several of us have cleanups or optimization that we want to
> follow (I had one that inlines what isolate_migratepages_block() wanted
> of __isolate_lru_page() into that function, so simplifying what vmscan.c
> needs; perhaps that can now eliminate it completely, I've not tried
> recently).  But there was a point at which the series was growing
> ten patches per release as we all added our bits and pieces on top,
> it got harder and harder to review the whole, and further from
> getting the basics in: I do push back against that tendency.
> 
> Hugh
>