Tang Chen <tangchen@xxxxxxxxxxxxxx> writes: > AIO ring page migration has been implemented by the following patch: > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/fs/aio.c?id=36bc08cc01709b4a9bb563b35aa530241ddc63e3 > > In this patch, ctx->completion_lock is used to prevent other processes > from accessing the ring page being migrated. > > But in aio_setup_ring(), ioctx_add_table() and aio_read_events_ring(), > when writing to the ring page, they didn't take ctx->completion_lock. > > As a result, for example, we have the following problem: > > thread 1 | thread 2 > | > aio_migratepage() | > |-> take ctx->completion_lock | > |-> migrate_page_copy(new, old) | > | *NOW*, ctx->ring_pages[idx] == old | > | > | *NOW*, ctx->ring_pages[idx] == old > | aio_read_events_ring() > | |-> ring = kmap_atomic(ctx->ring_pages[0]) > | |-> ring->head = head; *HERE, write to the old ring page* > | |-> kunmap_atomic(ring); > | > |-> ctx->ring_pages[idx] = new | > | *BUT NOW*, the content of | > | ring_pages[idx] is old. | > |-> release ctx->completion_lock | > > As above, the new ring page will not be updated. > > The solution is taking ctx->completion_lock in thread 2, which means, > in aio_setup_ring(), ioctx_add_table() and aio_read_events_ring() when > writing to ring pages. Thanks for the good explanation and for adding comments to the code. This looks right to me. I guess the only issue I have with it is that the code paths changed don't run in interrupt context, so I think you could just use spin_lock_irq. That's not a big deal, though. Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx> > Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx> > Signed-off-by: Tang Chen <tangchen@xxxxxxxxxxxxxx> > --- > fs/aio.c | 33 +++++++++++++++++++++++++++++++++ > 1 file changed, 33 insertions(+) > > diff --git a/fs/aio.c b/fs/aio.c > index 062a5f6..50c089c 100644 > --- a/fs/aio.c > +++ b/fs/aio.c > @@ -366,6 +366,7 @@ static int aio_setup_ring(struct kioctx *ctx) > int nr_pages; > int i; > struct file *file; > + unsigned long flags; > > /* Compensate for the ring buffer's head/tail overlap entry */ > nr_events += 2; /* 1 is required, 2 for good luck */ > @@ -437,6 +438,14 @@ static int aio_setup_ring(struct kioctx *ctx) > ctx->user_id = ctx->mmap_base; > ctx->nr_events = nr_events; /* trusted copy */ > > + /* > + * The aio ring pages are user space pages, so they can be migrated. > + * When writing to an aio ring page, we should ensure the page is not > + * being migrated. Aio page migration procedure is protected by > + * ctx->completion_lock, so we add this lock here. > + */ > + spin_lock_irqsave(&ctx->completion_lock, flags); > + > ring = kmap_atomic(ctx->ring_pages[0]); > ring->nr = nr_events; /* user copy */ > ring->id = ~0U; > @@ -448,6 +457,8 @@ static int aio_setup_ring(struct kioctx *ctx) > kunmap_atomic(ring); > flush_dcache_page(ctx->ring_pages[0]); > > + spin_unlock_irqrestore(&ctx->completion_lock, flags); > + > return 0; > } > > @@ -542,6 +553,7 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm) > unsigned i, new_nr; > struct kioctx_table *table, *old; > struct aio_ring *ring; > + unsigned long flags; > > spin_lock(&mm->ioctx_lock); > rcu_read_lock(); > @@ -556,9 +568,19 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm) > rcu_read_unlock(); > spin_unlock(&mm->ioctx_lock); > > + /* > + * Accessing ring pages must be done > + * holding ctx->completion_lock to > + * prevent aio ring page migration > + * procedure from migrating ring pages. > + */ > + spin_lock_irqsave(&ctx->completion_lock, > + flags); > ring = kmap_atomic(ctx->ring_pages[0]); > ring->id = ctx->id; > kunmap_atomic(ring); > + spin_unlock_irqrestore( > + &ctx->completion_lock, flags); > return 0; > } > > @@ -1021,6 +1043,7 @@ static long aio_read_events_ring(struct kioctx *ctx, > unsigned head, tail, pos; > long ret = 0; > int copy_ret; > + unsigned long flags; > > mutex_lock(&ctx->ring_lock); > > @@ -1066,11 +1089,21 @@ static long aio_read_events_ring(struct kioctx *ctx, > head %= ctx->nr_events; > } > > + /* > + * The aio ring pages are user space pages, so they can be migrated. > + * When writing to an aio ring page, we should ensure the page is not > + * being migrated. Aio page migration procedure is protected by > + * ctx->completion_lock, so we add this lock here. > + */ > + spin_lock_irqsave(&ctx->completion_lock, flags); > + > ring = kmap_atomic(ctx->ring_pages[0]); > ring->head = head; > kunmap_atomic(ring); > flush_dcache_page(ctx->ring_pages[0]); > > + spin_unlock_irqrestore(&ctx->completion_lock, flags); > + > pr_debug("%li h%u t%u\n", ret, head, tail); > > put_reqs_available(ctx, ret); -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
