linux-kernel - Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

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Date:   Thu, 25 Aug 2022 11:04:59 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Nadav Amit <nadav.amit@...il.com>,
        huang ying <huang.ying.caritas@...il.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, paulus@...abs.org,
        linuxppc-dev@...ts.ozlabs.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/migrate_device.c: Copy pte dirty bit to page

On Thu, Aug 25, 2022 at 11:24:03AM +1000, Alistair Popple wrote:
> By the way it's still an optimisation because in most cases we can avoid
> calling try_to_migrate() and walking the rmap altogether if we install
> the migration entries here. But I agree the comment is misleading.

There's one follow up question I forgot to ask on the trylock thing.  I
figured maybe I should ask out loud since we're at it.

Since migrate_vma_setup() always only use trylock (even if before dropping
the prepare() code), does it mean that it can randomly fail?

I looked at some of the callers, it seems not all of them are ready to
handle that (__kvmppc_svm_page_out() or svm_migrate_vma_to_vram()).  Is it
safe?  Do the callers need to always properly handle that (unless the
migration is only a best-effort, but it seems not always the case).

Besides, since I read the old code of prepare(), I saw this comment:

-		if (!(migrate->src[i] & MIGRATE_PFN_LOCKED)) {
-			/*
-			 * Because we are migrating several pages there can be
-			 * a deadlock between 2 concurrent migration where each
-			 * are waiting on each other page lock.
-			 *
-			 * Make migrate_vma() a best effort thing and backoff
-			 * for any page we can not lock right away.
-			 */
-			if (!trylock_page(page)) {
-				migrate->src[i] = 0;
-				migrate->cpages--;
-				put_page(page);
-				continue;
-			}
-			remap = false;
-			migrate->src[i] |= MIGRATE_PFN_LOCKED;
-		}

I'm a bit curious whether that deadlock mentioned in the comment is
observed in reality?

If the page was scanned in the same address space, logically the lock order
should be guaranteed (if both page A&B, both threads should lock in order).
I think the order can be changed if explicitly did so (e.g. fork() plus
mremap() for anonymous here) but I just want to make sure I get the whole
point of it.

Thanks,

-- 
Peter Xu