linux-kernel - Re: [PATCH] mm/gup.c: Simplify and fix check_and_migrate_movable

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Date:   Tue, 2 Aug 2022 09:21:08 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     akpm@...ux-foundation.org, minchan@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jhubbard@...dia.com,
        pasha.tatashin@...een.com
Subject: Re: [PATCH] mm/gup.c: Simplify and fix
 check_and_migrate_movable_pages() return codes

On Mon, Aug 01, 2022 at 12:18:53PM +1000, Alistair Popple wrote:

> > AFAICT there is no reason to 'continue' in most of these paths since
> > we intend to return to userspace with an error anyhow? Why try to
> > isolate more pages?
> 
> The main reason would be if callers want to retry the operation. AFAIK
> isolate_folio_lru() can have transient failures, so if callers want to
> retry it makes sense to isolate and migrate as many pages as possible
> rather than one page at a time as subsequent retries may find different
> pages that can't be isolated.

Except we don't try to do the migrate, we just isolate and then
unisolate and return failure.

> Actually I should have called this out more clearly - the previous
> behaviour on isolation failure was to retry indefinitely which is what
> lead to looping in the kernel. This patch turns isolation failure into
> an error and doesn't retry. I wonder though if we need to maintain a
> retry count similar to what migrate_pages() does if there are unexpected
> page refs?

This makes more sense, exporting this mess to the caller and hoping
they retry (they won't) doesn't make sense..

Jason