Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception path for isolate_lru_pages

From: Hugh Dickins
Date: Wed Sep 09 2020 - 17:05:41 EST

On Wed, 9 Sep 2020, Matthew Wilcox wrote:
> On Wed, Sep 09, 2020 at 11:24:14AM -0700, Hugh Dickins wrote:
> > After overnight reflection, my own preference would be simply to
> > drop this patch. I think we are making altogether too much of a
> > fuss here over what was simply correct as plain put_page()
> > (and further from correct if we change it to leak the page in an
> > unforeseen circumstance).
> >
> > And if Alex's comment was not quite grammatically correct, never mind,
> > it said as much as was worth saying. I got more worried by his
> > placement of the "busy:" label, but that does appear to work correctly.
> >
> > There's probably a thousand places where put_page() is used, where
> > it would be troublesome if it were the final put_page(): this one
> > bothered you because you'd been looking at isolate_migratepages_block(),
> > and its necessary avoidance of lru_lock recursion on put_page();
> > but let's just just leave this put_page() as is.
> My problem with put_page() is that it's no longer the simple
> decrement-and-branch-to-slow-path-if-zero that it used to be. It has the
> awful devmap excrement in it so it really expands into a lot of code.
> I really wish that "feature" could be backed out again. It clearly
> wasn't ready for merge.

And I suppose I should thank you for opening my eyes to that.
I knew there was "dev" stuff inside __put_page(), but didn't
realize that the inline put_page() has now been defiled.
Yes, I agree, that is horrid and begs to be undone.

But this is not the mail thread for discussing that, and we should
not use strange alternatives to put_page(), here or elsewhere,
just to avoid that (surely? hopefully?) temporary excrescence.