linux-kernel - Re: [PATCH v3 4/4] ceph: assert writeback loop invariants

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Message-ID: <345540f12f8825ebbe067564fbd94bc1966e126b.camel@ibm.com>
Date: Mon, 26 Jan 2026 22:54:16 +0000
From: Viacheslav Dubeyko <Slava.Dubeyko@....com>
To: Xiubo Li <xiubli@...hat.com>, "idryomov@...il.com" <idryomov@...il.com>,
        "cfsworks@...il.com" <cfsworks@...il.com>
CC: Milind Changire <mchangir@...hat.com>,
        "ceph-devel@...r.kernel.org"
	<ceph-devel@...r.kernel.org>,
        "brauner@...nel.org" <brauner@...nel.org>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re:  [PATCH v3 4/4] ceph: assert writeback loop invariants

On Sun, 2026-01-25 at 18:30 -0800, Sam Edwards wrote:
> If `locked_pages` is zero, the page array must not be allocated:
> ceph_process_folio_batch() uses `locked_pages` to decide when to
> allocate `pages`, and redundant allocations trigger
> ceph_allocate_page_array()'s BUG_ON(), resulting in a worker oops (and
> writeback stall) or even a kernel panic. Consequently, the main loop in
> ceph_writepages_start() assumes that the lifetime of `pages` is confined
> to a single iteration.
> 
> This expectation is currently not clear enough, as evidenced by two
> recent patches which fix oopses caused by `pages` persisting into
> the next loop iteration:
> - "ceph: do not propagate page array emplacement errors as batch errors"
> - "ceph: free page array when ceph_submit_write() fails"
> 
> Use an explicit BUG_ON() at the top of the loop to assert the loop's
> preexisting expectation that `pages` is cleaned up by the previous
> iteration. Because this is closely tied to `locked_pages`, also make it
> the previous iteration's responsibility to guarantee its reset, and
> verify with a second new BUG_ON() instead of handling (and masking)
> failures to do so.
> 
> This patch does not change invariants, behavior, or failure modes.
> The added BUG_ON() lines catch conditions that would already trigger oops,
> but do so earlier for easier debugging and programmer clarity.
> 
> Signed-off-by: Sam Edwards <CFSworks@...il.com>
> ---
>  fs/ceph/addr.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index cdf11288d6b7..4e392fc70d33 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1663,7 +1663,9 @@ static int ceph_writepages_start(struct address_space *mapping,
>  		tag_pages_for_writeback(mapping, ceph_wbc.index, ceph_wbc.end);
>  
>  	while (!has_writeback_done(&ceph_wbc)) {
> -		ceph_wbc.locked_pages = 0;
> +		BUG_ON(ceph_wbc.locked_pages);
> +		BUG_ON(ceph_wbc.pages);

My complains are still the same. I would like not have BUG_ON() here.

Thanks,
Slava.

> +
>  		ceph_wbc.max_pages = ceph_wbc.wsize >> PAGE_SHIFT;
>  
>  get_more_pages: