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

[Matthew Wilcox]

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.