linux-kernel - Re: [PATCH 2/3] mm: drop VMA lock before waiting for migration

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Date:   Tue, 2 May 2023 15:28:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...e.com,
        josef@...icpanda.com, jack@...e.cz, ldufour@...ux.ibm.com,
        laurent.dufour@...ibm.com, michel@...pinasse.org,
        liam.howlett@...cle.com, jglisse@...gle.com, vbabka@...e.cz,
        minchan@...gle.com, dave@...olabs.net, punit.agrawal@...edance.com,
        lstoakes@...il.com, hdanton@...a.com, apopple@...dia.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 2/3] mm: drop VMA lock before waiting for migration

On Mon, May 01, 2023 at 10:50:24AM -0700, Suren Baghdasaryan wrote:
> migration_entry_wait does not need VMA lock, therefore it can be dropped
> before waiting. Introduce VM_FAULT_VMA_UNLOCKED to indicate that VMA
> lock was dropped while in handle_mm_fault().
> Note that once VMA lock is dropped, the VMA reference can't be used as
> there are no guarantees it was not freed.

How about we introduce:

void vmf_end_read(struct vm_fault *vmf)
{
	if (!vmf->vma)
		return;
	vma_end_read(vmf->vma);
	vmf->vma = NULL;
}

Now we don't need a new flag, and calling vmf_end_read() is idempotent.

Oh, argh, we create the vmf too late.  We really need to hoist the
creation of vm_fault to the callers of handle_mm_fault().