On Fri, Mar 10, 2017 at 10:15 AM, Shaohua Li <shli@xxxxxxxxxx> wrote:
> On Fri, Mar 10, 2017 at 04:16:58PM +0100, Artur Paszkiewicz wrote:
>> On 03/10/2017 12:24 AM, Shaohua Li wrote:
>> > On Thu, Mar 09, 2017 at 09:59:59AM +0100, Artur Paszkiewicz wrote:
>> >> Implement the calculation of partial parity for a stripe and PPL write
>> >> logging functionality. The description of PPL is added to the
>> >> documentation. More details can be found in the comments in raid5-ppl.c.
>> >>
>> >> Attach a page for holding the partial parity data to stripe_head.
>> >> Allocate it only if mddev has the MD_HAS_PPL flag set.
>> >>
>> >> Partial parity is the xor of not modified data chunks of a stripe and is
>> >> calculated as follows:
>> >>
>> >> - reconstruct-write case:
>> >> xor data from all not updated disks in a stripe
>> >>
>> >> - read-modify-write case:
>> >> xor old data and parity from all updated disks in a stripe
>> >>
>> >> Implement it using the async_tx API and integrate into raid_run_ops().
>> >> It must be called when we still have access to old data, so do it when
>> >> STRIPE_OP_BIODRAIN is set, but before ops_run_prexor5(). The result is
>> >> stored into sh->ppl_page.
>> >>
>> >> Partial parity is not meaningful for full stripe write and is not stored
>> >> in the log or used for recovery, so don't attempt to calculate it when
>> >> stripe has STRIPE_FULL_WRITE.
>> >>
>> >> Put the PPL metadata structures to md_p.h because userspace tools
>> >> (mdadm) will also need to read/write PPL.
>> >>
>> >> Warn about using PPL with enabled disk volatile write-back cache for
>> >> now. It can be removed once disk cache flushing before writing PPL is
>> >> implemented.
>> >>
>> >> Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@xxxxxxxxx>
>> >
>> > ... snip ...
>> >
>> >> +struct dma_async_tx_descriptor *
>> >> +ops_run_partial_parity(struct stripe_head *sh, struct raid5_percpu *percpu,
>> >> + struct dma_async_tx_descriptor *tx)
>> >> +{
>> >> + int disks = sh->disks;
>> >> + struct page **xor_srcs = flex_array_get(percpu->scribble, 0);
>> >> + int count = 0, pd_idx = sh->pd_idx, i;
>> >> + struct async_submit_ctl submit;
>> >> +
>> >> + pr_debug("%s: stripe %llu\n", __func__, (unsigned long long)sh->sector);
>> >> +
>> >> + /*
>> >> + * Partial parity is the XOR of stripe data chunks that are not changed
>> >> + * during the write request. Depending on available data
>> >> + * (read-modify-write vs. reconstruct-write case) we calculate it
>> >> + * differently.
>> >> + */
>> >> + if (sh->reconstruct_state == reconstruct_state_prexor_drain_run) {
>> >> + /* rmw: xor old data and parity from updated disks */
>> >> + for (i = disks; i--;) {
>> >> + struct r5dev *dev = &sh->dev[i];
>> >> + if (test_bit(R5_Wantdrain, &dev->flags) || i == pd_idx)
>> >> + xor_srcs[count++] = dev->page;
>> >> + }
>> >> + } else if (sh->reconstruct_state == reconstruct_state_drain_run) {
>> >> + /* rcw: xor data from all not updated disks */
>> >> + for (i = disks; i--;) {
>> >> + struct r5dev *dev = &sh->dev[i];
>> >> + if (test_bit(R5_UPTODATE, &dev->flags))
>> >> + xor_srcs[count++] = dev->page;
>> >> + }
>> >> + } else {
>> >> + return tx;
>> >> + }
>> >> +
>> >> + init_async_submit(&submit, ASYNC_TX_XOR_ZERO_DST, tx, NULL, sh,
>> >> + flex_array_get(percpu->scribble, 0)
>> >> + + sizeof(struct page *) * (sh->disks + 2));
>> >
>> > Since this should be done before biodrain, should this add ASYNC_TX_FENCE flag?
>>
>> The result of this calculation isn't used later by other async_tx
>> operations, so it's not needed here, if I understand this correctly. But
>> maybe later we could optimize and use partial parity to calculate full
>> parity, then it will be necessary.
>
> But the source pages will be overwritten soon, if no fence, I think this is a
> problem. Anyway, I'll let Dan to clarify.
>
> Dan,
> We are using async API for below operations:
> 1. xor several source pages to a dest page
> 2. memcpy other data to the source pages
>
> The two operations will be in an async chain. Should the first operation
> include ASYNC_TX_FENCE flag? The API comment declares the flag is required if
> the destination page is used by subsequent operations, but I suspect it should
> be used too if the subsequent operations could change the source pages.
I think you're right. If operation2 could change operation1 source
data we should include the fence.
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