• Jens Axboe's avatar
    block: reduce stack footprint of blk_recount_segments() · 1e428079
    Jens Axboe authored
    blk_recalc_rq_segments() requires a request structure passed in, which
    we don't have from blk_recount_segments(). So the latter allocates one on
    the stack, using > 400 bytes of stack for that. This can cause us to spill
    over one page of stack from ext4 at least:
    
     0)     4560     400   blk_recount_segments+0x43/0x62
     1)     4160      32   bio_phys_segments+0x1c/0x24
     2)     4128      32   blk_rq_bio_prep+0x2a/0xf9
     3)     4096      32   init_request_from_bio+0xf9/0xfe
     4)     4064     112   __make_request+0x33c/0x3f6
     5)     3952     144   generic_make_request+0x2d1/0x321
     6)     3808      64   submit_bio+0xb9/0xc3
     7)     3744      48   submit_bh+0xea/0x10e
     8)     3696     368   ext4_mb_init_cache+0x257/0xa6a [ext4]
     9)     3328     288   ext4_mb_regular_allocator+0x421/0xcd9 [ext4]
    10)     3040     160   ext4_mb_new_blocks+0x211/0x4b4 [ext4]
    11)     2880     336   ext4_ext_get_blocks+0xb61/0xd45 [ext4]
    12)     2544      96   ext4_get_blocks_wrap+0xf2/0x200 [ext4]
    13)     2448      80   ext4_da_get_block_write+0x6e/0x16b [ext4]
    14)     2368     352   mpage_da_map_blocks+0x7e/0x4b3 [ext4]
    15)     2016     352   ext4_da_writepages+0x2ce/0x43c [ext4]
    16)     1664      32   do_writepages+0x2d/0x3c
    17)     1632     144   __writeback_single_inode+0x162/0x2cd
    18)     1488      96   generic_sync_sb_inodes+0x1e3/0x32b
    19)     1392      16   sync_sb_inodes+0xe/0x10
    20)     1376      48   writeback_inodes+0x69/0xb3
    21)     1328     208   balance_dirty_pages_ratelimited_nr+0x187/0x2f9
    22)     1120     224   generic_file_buffered_write+0x1d4/0x2c4
    23)      896     176   __generic_file_aio_write_nolock+0x35f/0x393
    24)      720      80   generic_file_aio_write+0x6c/0xc8
    25)      640      80   ext4_file_write+0xa9/0x137 [ext4]
    26)      560     320   do_sync_write+0xf0/0x137
    27)      240      48   vfs_write+0xb3/0x13c
    28)      192      64   sys_write+0x4c/0x74
    29)      128     128   system_call_fastpath+0x16/0x1b
    
    Split the segment counting out into a __blk_recalc_rq_segments() helper
    to avoid allocating an onstack request just for checking the physical
    segment count.
    Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
    1e428079
blk-merge.c 10.1 KB