On Thu, 16 Nov 2006 12:15:16 -0800 Mingming Cao <cmm@xxxxxxxxxx> wrote: > On Thu, 2006-11-16 at 01:13 -0800, Andrew Morton wrote: > > On Thu, 16 Nov 2006 00:49:20 -0800 > > Mingming Cao <cmm@xxxxxxxxxx> wrote: > > > > > On Wed, 2006-11-15 at 23:22 -0800, Andrew Morton wrote: > > > > On Wed, 15 Nov 2006 22:55:43 -0800 > > > > Mingming Cao <cmm@xxxxxxxxxx> wrote: > > > > > > > > > Hmm, maxblocks, in bitmap_search_next_usable_block(), is the end block > > > > > number of the range to search, not the lengh of the range. maxblocks > > > > > get passed to ext2_find_next_zero_bit(), where it expecting to take the > > > > > _size_ of the range to search instead... > > > > > > > > > > Something like this: (this is not a patch) > > > > > @@ -524,7 +524,7 @@ bitmap_search_next_usable_block(ext2_grp > > > > > ext2_grpblk_t next; > > > > > > > > > > - next = ext2_find_next_zero_bit(bh->b_data, maxblocks, start); > > > > > + next = ext2_find_next_zero_bit(bh->b_data, maxblocks-start + 1, start); > > > > > if (next >= maxblocks) > > > > > return -1; > > > > > return next; > > > > > } > > > > > > > > yes, the `size' arg to find_next_zero_bit() represents the number of bits > > > > to scan at `offset'. > > > > > > > > So I think your change is correctish. But we don't want the "+ 1", do we? > > > > > > > I think we still need the "+1", maxblocks here is the ending block of > > > the reservation window, so the number of bits to scan =end-start+1. > > > > > > > If we're right then this bug could cause the code to scan off the end of the > > > > bitmap. But it won't explain Hugh's bug, because of the if (next >= maxblocks). > > > > > > > > > > Yeah.. at first I thought it might be related, then, thinked it over, > > > the bug only makes the bits to scan larger, so if find_next_zero_bit() > > > returns something off the end of bitmap, that is fine, it just > > > indicating that there is no free bit left in the rest of bitmap, which > > > is expected behavior. So bitmap_search_next_usable_block() fail is the > > > expected. It will move on to next block group and try to create a new > > > reservation window there. > > > > I wonder why it's never oopsed. Perhaps there's always a zero in there for > > some reason. > > > > Why you think it should oopsed? Even if find_next_zero_bit() finds a > zero bit beyond of the end of bitmap, the check next > maxblocks will > catch this and make sure we are not taking a zero bit out of the bitmap > range, so it fails as expected. If it can read off the end of the buffer, it can oops. With CONFIG_DEBUG_PAGEALLOC, especially. > > > That does not explain the repeated reservation window add and remove > > > behavior Huge has reported. > > > > I spent quite some time comparing with ext3. I'm a bit stumped and I'm > > suspecting that the simplistic porting the code is now OK, but something's > > just wrong. > > > > I assume that the while (1) loop in ext3_try_to_allocate_with_rsv() has > > gone infinite. I don't see why, but more staring is needed. > > > > The loop should not go forever, it will stops when there is no window > with free bit to reserve in the given block group. It seems to have done so in Hugh's testing, but there's some question there now. Although I didn't check to see if there's a significant difference between Hugh's patch and mine. > > What lock protects the fields in struct ext[234]_reserve_window from being > > concurrently modified by two CPUs? None, it seems. Ditto > > ext[234]_reserve_window_node. i_mutex will cover it for write(), but not > > for pageout over a file hole. If we end up with a zero- or negative-sized > > window then odd things might happen. > > > > Yes, trucate_mutex protect both struct ext[234]_reserve_window and ext > [234]_reserve_window_node, and struct ext[234]_block_alloc_info. > Actually I think truncate_mutex protects all data structures related to > block allocation/mapping structures. Yes. I guess ext2 needs a new mutex for this. Sad. - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
