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Date: Sat, 30 Mar 2024 13:55:29 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Lorenzo Stoakes <lstoakes@...il.com>,
Christoph Hellwig <hch@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
Dave Chinner <david@...morbit.com>,
Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
Jens Axboe <axboe@...nel.dk>, Omar Sandoval <osandov@...com>
Subject: Re: [PATCH 1/1] mm: vmalloc: Fix lockdep warning
On Fri, Mar 29, 2024 at 03:44:40PM +0800, Baoquan He wrote:
> On 03/28/24 at 03:03pm, Uladzislau Rezki (Sony) wrote:
> > A lockdep reports a possible deadlock in the find_vmap_area_exceed_addr_lock()
> > function:
> >
> > ============================================
> > WARNING: possible recursive locking detected
> > 6.9.0-rc1-00060-ged3ccc57b108-dirty #6140 Not tainted
> > --------------------------------------------
> > drgn/455 is trying to acquire lock:
> > ffff0000c00131d0 (&vn->busy.lock/1){+.+.}-{2:2}, at: find_vmap_area_exceed_addr_lock+0x64/0x124
> >
> > but task is already holding lock:
> > ffff0000c0011878 (&vn->busy.lock/1){+.+.}-{2:2}, at: find_vmap_area_exceed_addr_lock+0x64/0x124
> >
> > other info that might help us debug this:
> > Possible unsafe locking scenario:
> >
> > CPU0
> > ----
> > lock(&vn->busy.lock/1);
> > lock(&vn->busy.lock/1);
> >
> > *** DEADLOCK ***
> >
> > indeed it can happen if the find_vmap_area_exceed_addr_lock()
> > gets called concurrently because it tries to acquire two nodes
> > locks. It was done to prevent removing a lowest VA found on a
> > previous step.
> >
> > To address this a lowest VA is found first without holding a
> > node lock where it resides. As a last step we check if a VA
> > still there because it can go away, if removed, proceed with
> > next lowest.
> >
> > Fixes: 53becf32aec1 ("mm: vmalloc: support multiple nodes in vread_iter")
> > Tested-by: Jens Axboe <axboe@...nel.dk>
> > Tested-by: Omar Sandoval <osandov@...com>
> > Reported-by: Jens Axboe <axboe@...nel.dk>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > ---
> > mm/vmalloc.c | 74 +++++++++++++++++++++++++++++++---------------------
> > 1 file changed, 44 insertions(+), 30 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index e94ce4562805..a5a5dfc3843e 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -989,6 +989,27 @@ unsigned long vmalloc_nr_pages(void)
> > return atomic_long_read(&nr_vmalloc_pages);
> > }
> >
> > +static struct vmap_area *__find_vmap_area(unsigned long addr, struct rb_root *root)
> > +{
> > + struct rb_node *n = root->rb_node;
> > +
> > + addr = (unsigned long)kasan_reset_tag((void *)addr);
> > +
> > + while (n) {
> > + struct vmap_area *va;
> > +
> > + va = rb_entry(n, struct vmap_area, rb_node);
> > + if (addr < va->va_start)
> > + n = n->rb_left;
> > + else if (addr >= va->va_end)
> > + n = n->rb_right;
> > + else
> > + return va;
> > + }
> > +
> > + return NULL;
> > +}
> > +
> > /* Look up the first VA which satisfies addr < va_end, NULL if none. */
> > static struct vmap_area *
> > __find_vmap_area_exceed_addr(unsigned long addr, struct rb_root *root)
> > @@ -1025,47 +1046,40 @@ __find_vmap_area_exceed_addr(unsigned long addr, struct rb_root *root)
> > static struct vmap_node *
> > find_vmap_area_exceed_addr_lock(unsigned long addr, struct vmap_area **va)
> > {
> > - struct vmap_node *vn, *va_node = NULL;
> > - struct vmap_area *va_lowest;
> > + unsigned long va_start_lowest;
> > + struct vmap_node *vn;
> > int i;
> >
> > - for (i = 0; i < nr_vmap_nodes; i++) {
> > +repeat:
> > + for (i = 0, va_start_lowest = 0; i < nr_vmap_nodes; i++) {
> > vn = &vmap_nodes[i];
> >
> > spin_lock(&vn->busy.lock);
> > - va_lowest = __find_vmap_area_exceed_addr(addr, &vn->busy.root);
> > - if (va_lowest) {
> > - if (!va_node || va_lowest->va_start < (*va)->va_start) {
> > - if (va_node)
> > - spin_unlock(&va_node->busy.lock);
> > -
> > - *va = va_lowest;
> > - va_node = vn;
> > - continue;
> > - }
> > - }
> > + *va = __find_vmap_area_exceed_addr(addr, &vn->busy.root);
> > +
> > + if (*va)
> > + if (!va_start_lowest || (*va)->va_start < va_start_lowest)
> > + va_start_lowest = (*va)->va_start;
>
> How about below change about va_start_lowest? Personal preference, not
> strong opinion.
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 9b1a41e12d70..bd6a66c54ad2 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1046,19 +1046,19 @@ __find_vmap_area_exceed_addr(unsigned long addr, struct rb_root *root)
> static struct vmap_node *
> find_vmap_area_exceed_addr_lock(unsigned long addr, struct vmap_area **va)
> {
> - unsigned long va_start_lowest;
> + unsigned long va_start_lowest = ULONG_MAX;
> struct vmap_node *vn;
> int i;
>
> repeat:
> - for (i = 0, va_start_lowest = 0; i < nr_vmap_nodes; i++) {
> + for (i = 0; i < nr_vmap_nodes; i++) {
> vn = &vmap_nodes[i];
>
> spin_lock(&vn->busy.lock);
> *va = __find_vmap_area_exceed_addr(addr, &vn->busy.root);
>
> if (*va)
> - if (!va_start_lowest || (*va)->va_start < va_start_lowest)
> + if ((*va)->va_start < va_start_lowest)
> va_start_lowest = (*va)->va_start;
> spin_unlock(&vn->busy.lock);
> }
> @@ -1069,7 +1069,7 @@ find_vmap_area_exceed_addr_lock(unsigned long addr, struct vmap_area **va)
> * been removed concurrently thus we need to proceed
> * with next one what is a rare case.
> */
> - if (va_start_lowest) {
> + if (va_start_lowest != ULONG_MAX) {
> vn = addr_to_node(va_start_lowest);
>
> spin_lock(&vn->busy.lock);
>
>
To me it looks as incomplete. The "va_start_lowest" should be initialized
when repeat. Otherwise we can end up with an infinite repeating because
va_start_lowest != ULONG_MAX.
> > }
> >
> > - return va_node;
> > -}
> > -
> > -static struct vmap_area *__find_vmap_area(unsigned long addr, struct rb_root *root)
> > -{
> > - struct rb_node *n = root->rb_node;
> > + /*
> > + * Check if found VA exists, it might it is gone away.
> ~~~~ grammer mistake?
> > + * In this case we repeat the search because a VA has
> > + * been removed concurrently thus we need to proceed
> > + * with next one what is a rare case.
> ~~~~ typo, which?
> > + */
> > + if (va_start_lowest) {
> > + vn = addr_to_node(va_start_lowest);
> >
> > - addr = (unsigned long)kasan_reset_tag((void *)addr);
> > + spin_lock(&vn->busy.lock);
> > + *va = __find_vmap_area(va_start_lowest, &vn->busy.root);
> >
> > - while (n) {
> > - struct vmap_area *va;
> > + if (*va)
> > + return vn;
> >
> > - va = rb_entry(n, struct vmap_area, rb_node);
> > - if (addr < va->va_start)
> > - n = n->rb_left;
> > - else if (addr >= va->va_end)
> > - n = n->rb_right;
> > - else
> > - return va;
> > + spin_unlock(&vn->busy.lock);
> > + goto repeat;
> > }
>
> Other than above nickpick concerns, this looks good to me.
>
> Reviewed-by: Baoquan He <bhe@...hat.com>
>
Thank you!
--
Uladzislau Rezki
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