Description of problem:

I found a problem about kswapd. This problem causes a system stall.

int try_to_free_pages(unsigned int gfp_mask)
{
        int ret = 1;

        gfp_mask = pf_gfp_mask(gfp_mask);
        if (gfp_mask & __GFP_WAIT) {
                current->flags |= PF_MEMALLOC;
                ret = do_try_to_free_pages(gfp_mask);
                current->flags &= ~PF_MEMALLOC;
        }

        return ret;
}

If kswapd call try_to_free_pages(), the PF_MEMALLOC flag is lost.
Under heavy memory pressure, kswapd may fail to call getblk() 
and call free_more_memory().
The free_more_memory() calls try_to_free_pages().
As a result, the kswapd loses the PF_MEMALLOC flag.

diff -uNr linux.org/mm/vmscan.c linux/mm/vmscan.c
--- linux.org/mm/vmscan.c       2004-12-13 14:46:03.000000000 +0900
+++ linux/mm/vmscan.c   2004-12-13 14:52:22.000000000 +0900
@@ -1273,6 +1273,7 @@
 int try_to_free_pages(unsigned int gfp_mask)
 {
        int ret = 1;
+       unsigned long pf_flags = current->flags & PF_MEMALLOC;

        gfp_mask = pf_gfp_mask(gfp_mask);
        if (gfp_mask & __GFP_WAIT) {
@@ -1281,6 +1282,8 @@
                current->flags &= ~PF_MEMALLOC;
        }

+        current->flags |= pf_flags;
+
        return ret;
 }


Version-Release number of selected component (if applicable):
kernel-2.4.21-20.EL

How reproducible:
It needs heavy memory pressure.
It is difficult to reproduce this problem.