On Tue, Apr 13, 2010 at 08:34:29PM +0100, Mel Gorman wrote: > > This problem is not a filesystem recursion problem which is, as I > > understand it, what GFP_NOFS is used to prevent. It's _any_ kernel > > code that uses signficant stack before trying to allocate memory > > that is the problem. e.g a select() system call: > > > > Depth Size Location (47 entries) > > ----- ---- -------- > > 0) 7568 16 mempool_alloc_slab+0x16/0x20 > > 1) 7552 144 mempool_alloc+0x65/0x140 > > 2) 7408 96 get_request+0x124/0x370 > > 3) 7312 144 get_request_wait+0x29/0x1b0 > > 4) 7168 96 __make_request+0x9b/0x490 > > 5) 7072 208 generic_make_request+0x3df/0x4d0 > > 6) 6864 80 submit_bio+0x7c/0x100 > > 7) 6784 96 _xfs_buf_ioapply+0x128/0x2c0 [xfs] > > .... > > 32) 3184 64 xfs_vm_writepage+0xab/0x160 [xfs] > > 33) 3120 384 shrink_page_list+0x65e/0x840 > > 34) 2736 528 shrink_zone+0x63f/0xe10 > > 35) 2208 112 do_try_to_free_pages+0xc2/0x3c0 > > 36) 2096 128 try_to_free_pages+0x77/0x80 > > 37) 1968 240 __alloc_pages_nodemask+0x3e4/0x710 > > 38) 1728 48 alloc_pages_current+0x8c/0xe0 > > 39) 1680 16 __get_free_pages+0xe/0x50 > > 40) 1664 48 __pollwait+0xca/0x110 > > 41) 1616 32 unix_poll+0x28/0xc0 > > 42) 1584 16 sock_poll+0x1d/0x20 > > 43) 1568 912 do_select+0x3d6/0x700 > > 44) 656 416 core_sys_select+0x18c/0x2c0 > > 45) 240 112 sys_select+0x4f/0x110 > > 46) 128 128 system_call_fastpath+0x16/0x1b > > > > There's 1.6k of stack used before memory allocation is called, 3.1k > > used there before ->writepage is entered, XFS used 3.5k, and > > if the mempool needed to allocate a page it would have blown the > > stack. If there was any significant storage subsystem (add dm, md > > and/or scsi of some kind), it would have blown the stack. > > > > Basically, there is not enough stack space available to allow direct > > reclaim to enter ->writepage _anywhere_ according to the stack usage > > profiles we are seeing here.... > > > > I'm not denying the evidence but how has it been gotten away with for years > then? Prevention of writeback isn't the answer without figuring out how > direct reclaimers can queue pages for IO and in the case of lumpy reclaim > doing sync IO, then waiting on those pages. So, I've been reading along, nodding my head to Dave's side of things because seeks are evil and direct reclaim makes seeks. I'd really loev for direct reclaim to somehow trigger writepages on large chunks instead of doing page by page spatters of IO to the drive. But, somewhere along the line I overlooked the part of Dave's stack trace that said: 43) 1568 912 do_select+0x3d6/0x700 Huh, 912 bytes...for select, really? From poll.h: /* ~832 bytes of stack space used max in sys_select/sys_poll before allocating additional memory. */ #define MAX_STACK_ALLOC 832 #define FRONTEND_STACK_ALLOC 256 #define SELECT_STACK_ALLOC FRONTEND_STACK_ALLOC #define POLL_STACK_ALLOC FRONTEND_STACK_ALLOC #define WQUEUES_STACK_ALLOC (MAX_STACK_ALLOC - FRONTEND_STACK_ALLOC) #define N_INLINE_POLL_ENTRIES (WQUEUES_STACK_ALLOC / sizeof(struct poll_table_entry)) So, select is intentionally trying to use that much stack. It should be using GFP_NOFS if it really wants to suck down that much stack...if only the kernel had some sort of way to dynamically allocate ram, it could try that too. -chris -- 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
