linux-kernel - Re: [PATCH v7 08/17] mm: allow vma_start_read_locked/vma_start_read_locked

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Message-ID: <929630d5-3a11-47f3-ac4d-4f162a11bbdc@suse.cz>
Date: Tue, 7 Jan 2025 18:28:33 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc: peterz@...radead.org, willy@...radead.org, liam.howlett@...cle.com,
 lorenzo.stoakes@...cle.com, mhocko@...e.com, hannes@...xchg.org,
 mjguzik@...il.com, oliver.sang@...el.com, mgorman@...hsingularity.net,
 david@...hat.com, peterx@...hat.com, oleg@...hat.com, dave@...olabs.net,
 paulmck@...nel.org, brauner@...nel.org, dhowells@...hat.com,
 hdanton@...a.com, hughd@...gle.com, lokeshgidra@...gle.com,
 minchan@...gle.com, jannh@...gle.com, shakeel.butt@...ux.dev,
 souravpanda@...gle.com, pasha.tatashin@...een.com, klarasmodin@...il.com,
 corbet@....net, linux-doc@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v7 08/17] mm: allow
 vma_start_read_locked/vma_start_read_locked_nested to fail

On 12/26/24 18:07, Suren Baghdasaryan wrote:
> With upcoming replacement of vm_lock with vm_refcnt, we need to handle a
> possibility of vma_start_read_locked/vma_start_read_locked_nested failing
> due to refcount overflow. Prepare for such possibility by changing these
> APIs and adjusting their users.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Lokesh Gidra <lokeshgidra@...gle.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

(I don't know if this can break some uffd users?)

> @@ -1483,10 +1484,16 @@ static int uffd_move_lock(struct mm_struct *mm,
>  	mmap_read_lock(mm);
>  	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
>  	if (!err) {
> -		vma_start_read_locked(*dst_vmap);
> -		if (*dst_vmap != *src_vmap)
> -			vma_start_read_locked_nested(*src_vmap,
> -						SINGLE_DEPTH_NESTING);
> +		if (vma_start_read_locked(*dst_vmap)) {
> +			if (*dst_vmap != *src_vmap) {
> +				if (!vma_start_read_locked_nested(*src_vmap,
> +							SINGLE_DEPTH_NESTING)) {
> +					vma_end_read(*dst_vmap);
> +					err = -EAGAIN;
> +				}
> +			}
> +		} else
> +			err = -EAGAIN;

Nit: { } per style

>  	}
>  	mmap_read_unlock(mm);
>  	return err;