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    SubjectRe: Bounce buffer deadlock

    > Yes. 2.4.6-pre8 fixes that (not sure if its up already).

    It is up.

    > > If the fix is to avoid page_launder in these cases then the number of
    > > occurrences when an alloc_pages fails will go up.
    > > I was attempting to come up with a way of making try_to_free_buffers
    > > fail on buffers which are being processed in the generic_make_request
    > > path by marking them, the problem is there is no single place to reset
    > > the state of a buffer so that try_to_free_buffers will wait for it.
    > > Doing it after the end of the loop in generic_make_request is race
    > > prone to say the least.
    > I really want to fix things like this in 2.5. (ie not avoid the deadlock
    > by completly avoiding physical IO, but avoid the deadlock by avoiding
    > physical IO on the "device" which is doing the allocation)
    > Could you send me your code ? No problem if it does not work at all :)

    Well, the basic idea is simple, but I suspect the implementation might
    rapidly become historical in 2.5. Basically I added a new buffer state bit,
    although BH_Req looks like it could be cannibalized, no one appears to check
    for it (is it really dead code?).

    Using a flag to skip buffers in try_to_free_buffers is easy:

    Index: linux/fs/buffer.c

    --- /usr/tmp/TmpDir.3237-0/linux/fs/buffer.c_1.68 Sat Jun 30 12:56:29 2001
    +++ linux/fs/buffer.c Sat Jun 30 12:57:52 2001
    @@ -2365,7 +2365,7 @@
    * Can the buffer be thrown out?
    -#define BUFFER_BUSY_BITS ((1<<BH_Dirty) | (1<<BH_Lock) | (1<<BH_Protected))
    +#define BUFFER_BUSY_BITS ((1<<BH_Dirty) | (1<<BH_Lock) | (1<<BH_Protected) | (1<<BH_Clamped))
    #define buffer_busy(bh) (atomic_read(&(bh)->b_count) | ((bh)->b_state & BUFFER_BUSY_BITS))

    @@ -2430,7 +2430,11 @@
    - if (wait) {
    + /* Buffers in the middle of generic_make_request processing cannot
    + * be waited for, they may be allocating memory right now and be
    + * locked by this thread.
    + */
    + if (wait && !buffer_clamped(tmp)) {
    sync_page_buffers(bh, wait);
    /* We waited synchronously, so we can free the buffers. */
    if (wait > 1 && !loop) {
    Index: linux/include/linux/fs.h
    --- /usr/tmp/TmpDir.3237-0/linux/include/linux/fs.h_1.99 Sat Jun 30 12:56:29 2001
    +++ linux/include/linux/fs.h Sat Jun 30 07:05:37 2001
    @@ -224,6 +224,8 @@
    BH_Mapped, /* 1 if the buffer has a disk mapping */
    BH_New, /* 1 if the buffer is new and not yet written out */
    BH_Protected, /* 1 if the buffer is protected */
    + BH_Clamped, /* 1 if the buffer cannot be reclaimed
    + * in it's current state */
    BH_Delay, /* 1 if the buffer is delayed allocate */

    BH_PrivateStart,/* not a state bit, but the first bit available
    @@ -286,6 +288,7 @@
    #define buffer_mapped(bh) __buffer_state(bh,Mapped)
    #define buffer_new(bh) __buffer_state(bh,New)
    #define buffer_protected(bh) __buffer_state(bh,Protected)
    +#define buffer_clamped(bh) __buffer_state(bh,Clamped)
    #define buffer_delay(bh) __buffer_state(bh,Delay)

    #define bh_offset(bh) ((unsigned long)(bh)->b_data & ~PAGE_MASK)

    The tricky part which I had not worked out how to do yet is to manage the
    clearing of a state bit in all the correct places. You would have to set it
    when the buffer got locked when I/O was about to start, it becomes clearable
    after the last memory allocation during the I/O submission process. I do
    not like the approach because there are so many ways a buffer can go
    once you get into generic_make_request. At first I thought I could just
    explicitly set and clear a flag around memory allocations like the bounce
    buffer path. However, that can lead to AB BA deadlocks between multiple
    threads submitting I/O requests. At this point I started to think I was
    going to build an unmaintainable rats nest and decided I had not got
    the correct answer.

    I am not sure that an approach which avoids a specific device will fly either,
    all the I/O can be on one device, and what does device mean when it comes
    to md/lvm and request remapping?


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