[bug report] xfs: use vmalloc instead of vm_map_area for buffer backing memory

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello Christoph Hellwig,

Commit e2874632a621 ("xfs: use vmalloc instead of vm_map_area for
buffer backing memory") from Mar 10, 2025 (linux-next), leads to the
following Smatch static checker warning:

	fs/xfs/xfs_buf.c:210 xfs_buf_free()
	warn: sleeping in atomic context

fs/xfs/xfs_buf.c
    196 static void
    197 xfs_buf_free(
    198         struct xfs_buf                *bp)
    199 {
    200         unsigned int                size = BBTOB(bp->b_length);
    201 
    202         trace_xfs_buf_free(bp, _RET_IP_);
    203 
    204         ASSERT(list_empty(&bp->b_lru));
    205 
    206         if (!xfs_buftarg_is_mem(bp->b_target) && size >= PAGE_SIZE)
    207                 mm_account_reclaimed_pages(howmany(size, PAGE_SHIFT));
    208 
    209         if (is_vmalloc_addr(bp->b_addr))
--> 210                 vfree(bp->b_addr);

vfree() can sleep.  Although it's fine to call it in interrupt context.

    211         else if (bp->b_flags & _XBF_KMEM)
    212                 kfree(bp->b_addr);
    213         else
    214                 folio_put(virt_to_folio(bp->b_addr));
    215 
    216         call_rcu(&bp->b_rcu, xfs_buf_free_callback);
    217 }

These warnings tend to have a lot of false positives because the call
tree is long.  There are two functions which call xfs_clear_li_failed()
while holding a spinlock.  These are the call trees.

xfs_trans_ail_delete() <- disables preempt
xfs_qm_dqflush_done() <- disables preempt
-> xfs_clear_li_failed()
   -> xfs_buf_rele()
      -> xfs_buf_rele_uncached()
         -> xfs_buf_free()

regards,
dan carpenter