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SubjectRe: [PATCH RFC] ashmem: Fix lockdep RECLAIM_FS false positive
On Mon, Feb 05, 2018 at 04:49:03PM -0800, Joel Fernandes wrote:

> [ 2115.359650] -(1)[106:kswapd0]=================================
> [ 2115.359665] -(1)[106:kswapd0][ INFO: inconsistent lock state ]
> [ 2115.359684] -(1)[106:kswapd0]4.9.60+ #2 Tainted: G W O
> [ 2115.359699] -(1)[106:kswapd0]---------------------------------
> [ 2115.359715] -(1)[106:kswapd0]inconsistent {RECLAIM_FS-ON-W} ->
> {IN-RECLAIM_FS-W} usage.

Please don't wrap log output, this is unreadable :/

Also, the output is from an ancient kernel and doesn't match the current
code.

> ---
> drivers/staging/android/ashmem.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
> index 372ce9913e6d..7e060f32aaa8 100644
> --- a/drivers/staging/android/ashmem.c
> +++ b/drivers/staging/android/ashmem.c
> @@ -32,6 +32,7 @@
> #include <linux/bitops.h>
> #include <linux/mutex.h>
> #include <linux/shmem_fs.h>
> +#include <linux/sched/mm.h>
> #include "ashmem.h"
>
> #define ASHMEM_NAME_PREFIX "dev/ashmem/"
> @@ -446,8 +447,17 @@ ashmem_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> if (!(sc->gfp_mask & __GFP_FS))
> return SHRINK_STOP;
>
> - if (!mutex_trylock(&ashmem_mutex))
> + /*
> + * Release reclaim-fs marking since we've already checked GFP_FS, This
> + * will prevent lockdep's reclaim recursion deadlock false positives.
> + * We'll renable it before returning from this function.
> + */
> + fs_reclaim_release(sc->gfp_mask);
> +
> + if (!mutex_trylock(&ashmem_mutex)) {
> + fs_reclaim_acquire(sc->gfp_mask);
> return -1;
> + }
>
> list_for_each_entry_safe(range, next, &ashmem_lru_list, lru) {
> loff_t start = range->pgstart * PAGE_SIZE;
> @@ -464,6 +474,8 @@ ashmem_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> break;
> }
> mutex_unlock(&ashmem_mutex);
> +
> + fs_reclaim_acquire(sc->gfp_mask);
> return freed;
> }

Yuck that is horrible.. so if GFP_FS was set, we bail, but if GFP_FS
wasn't set, why is fs_reclaim_*() doing anything at all?

That is, __need_fd_reclaim() should return false when !GFP_FS.

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