Re: [PATCH v2 35/49] nfs: add mirroring support to pgio layer

[Anna Schumaker <Anna.Schumaker@netapp.com>]

On 01/06/2015 01:27 PM, Weston Andros Adamson wrote:
> These issues are addressed and the comments are removed in subsequent patches
> from the same series.
> 
> Instead of having one huge patch that implements all of mirroring, I chose split
> it out into smaller patches. These notes were useful in making sure that the issues
> were addressed and should be useful as a guide to someone bisecting, etc.

Got it.  I'm still working my way through these patches, so I haven't seen the ones that remove the comments yet.

Thanks!
Anna

> 
> -dros
> 
> 
>> On Jan 6, 2015, at 1:11 PM, Anna Schumaker <Anna.Schumaker@netapp.com> wrote:
>>
>> Hey Dros and Tom,
>>
>> I see you're adding some new FIXME and TODOs in the comments.  Is there a plan for addressing these eventually?
>>
>> Thanks,
>> Anna
>>
>> On 12/24/2014 02:13 AM, Tom Haynes wrote:
>>> From: Weston Andros Adamson <dros@primarydata.com>
>>>
>>> This patch adds mirrored write support to the pgio layer. The default
>>> is to use one mirror, but pgio callers may define callbacks to change
>>> this to any value up to the (arbitrarily selected) limit of 16.
>>>
>>> The basic idea is to break out members of nfs_pageio_descriptor that cannot
>>> be shared between mirrored DSes and put them in a new structure.
>>>
>>> Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
>>> ---
>>> fs/nfs/direct.c              |  17 ++-
>>> fs/nfs/internal.h            |   1 +
>>> fs/nfs/objlayout/objio_osd.c |   3 +-
>>> fs/nfs/pagelist.c            | 270 +++++++++++++++++++++++++++++++++++--------
>>> fs/nfs/pnfs.c                |  26 +++--
>>> fs/nfs/read.c                |  30 ++++-
>>> fs/nfs/write.c               |  10 +-
>>> include/linux/nfs_page.h     |  20 +++-
>>> include/linux/nfs_xdr.h      |   1 +
>>> 9 files changed, 311 insertions(+), 67 deletions(-)
>>>
>>> diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
>>> index 1ee41d7..0178d4f 100644
>>> --- a/fs/nfs/direct.c
>>> +++ b/fs/nfs/direct.c
>>> @@ -360,8 +360,14 @@ static void nfs_direct_read_completion(struct nfs_pgio_header *hdr)
>>> 	spin_lock(&dreq->lock);
>>> 	if (test_bit(NFS_IOHDR_ERROR, &hdr->flags) && (hdr->good_bytes == 0))
>>> 		dreq->error = hdr->error;
>>> -	else
>>> -		dreq->count += hdr->good_bytes;
>>> +	else {
>>> +		/*
>>> +		 * FIXME: right now this only accounts for bytes written
>>> +		 *        to the first mirror
>>> +		 */
>>> +		if (hdr->pgio_mirror_idx == 0)
>>> +			dreq->count += hdr->good_bytes;
>>> +	}
>>> 	spin_unlock(&dreq->lock);
>>>
>>> 	while (!list_empty(&hdr->pages)) {
>>> @@ -724,7 +730,12 @@ static void nfs_direct_write_completion(struct nfs_pgio_header *hdr)
>>> 		dreq->error = hdr->error;
>>> 	}
>>> 	if (dreq->error == 0) {
>>> -		dreq->count += hdr->good_bytes;
>>> +		/*
>>> +		 * FIXME: right now this only accounts for bytes written
>>> +		 *        to the first mirror
>>> +		 */
>>> +		if (hdr->pgio_mirror_idx == 0)
>>> +			dreq->count += hdr->good_bytes;
>>> 		if (nfs_write_need_commit(hdr)) {
>>> 			if (dreq->flags == NFS_ODIRECT_RESCHED_WRITES)
>>> 				request_commit = true;
>>> diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
>>> index 05f9a87..ef1c703 100644
>>> --- a/fs/nfs/internal.h
>>> +++ b/fs/nfs/internal.h
>>> @@ -469,6 +469,7 @@ void nfs_init_cinfo(struct nfs_commit_info *cinfo,
>>> 		    struct nfs_direct_req *dreq);
>>> int nfs_key_timeout_notify(struct file *filp, struct inode *inode);
>>> bool nfs_ctx_key_to_expire(struct nfs_open_context *ctx);
>>> +void nfs_pageio_stop_mirroring(struct nfs_pageio_descriptor *pgio);
>>>
>>> #ifdef CONFIG_MIGRATION
>>> extern int nfs_migrate_page(struct address_space *,
>>> diff --git a/fs/nfs/objlayout/objio_osd.c b/fs/nfs/objlayout/objio_osd.c
>>> index d007780..9a5f2ee 100644
>>> --- a/fs/nfs/objlayout/objio_osd.c
>>> +++ b/fs/nfs/objlayout/objio_osd.c
>>> @@ -537,11 +537,12 @@ int objio_write_pagelist(struct nfs_pgio_header *hdr, int how)
>>> static size_t objio_pg_test(struct nfs_pageio_descriptor *pgio,
>>> 			  struct nfs_page *prev, struct nfs_page *req)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &pgio->pg_mirrors[pgio->pg_mirror_idx];
>>> 	unsigned int size;
>>>
>>> 	size = pnfs_generic_pg_test(pgio, prev, req);
>>>
>>> -	if (!size || pgio->pg_count + req->wb_bytes >
>>> +	if (!size || mirror->pg_count + req->wb_bytes >
>>> 	    (unsigned long)pgio->pg_layout_private)
>>> 		return 0;
>>>
>>> diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
>>> index 1c03187..eec12b7 100644
>>> --- a/fs/nfs/pagelist.c
>>> +++ b/fs/nfs/pagelist.c
>>> @@ -46,17 +46,22 @@ void nfs_pgheader_init(struct nfs_pageio_descriptor *desc,
>>> 		       struct nfs_pgio_header *hdr,
>>> 		       void (*release)(struct nfs_pgio_header *hdr))
>>> {
>>> -	hdr->req = nfs_list_entry(desc->pg_list.next);
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> +
>>> +	hdr->req = nfs_list_entry(mirror->pg_list.next);
>>> 	hdr->inode = desc->pg_inode;
>>> 	hdr->cred = hdr->req->wb_context->cred;
>>> 	hdr->io_start = req_offset(hdr->req);
>>> -	hdr->good_bytes = desc->pg_count;
>>> +	hdr->good_bytes = mirror->pg_count;
>>> 	hdr->dreq = desc->pg_dreq;
>>> 	hdr->layout_private = desc->pg_layout_private;
>>> 	hdr->release = release;
>>> 	hdr->completion_ops = desc->pg_completion_ops;
>>> 	if (hdr->completion_ops->init_hdr)
>>> 		hdr->completion_ops->init_hdr(hdr);
>>> +
>>> +	hdr->pgio_mirror_idx = desc->pg_mirror_idx;
>>> }
>>> EXPORT_SYMBOL_GPL(nfs_pgheader_init);
>>>
>>> @@ -480,7 +485,10 @@ nfs_wait_on_request(struct nfs_page *req)
>>> size_t nfs_generic_pg_test(struct nfs_pageio_descriptor *desc,
>>> 			   struct nfs_page *prev, struct nfs_page *req)
>>> {
>>> -	if (desc->pg_count > desc->pg_bsize) {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> +
>>> +	if (mirror->pg_count > mirror->pg_bsize) {
>>> 		/* should never happen */
>>> 		WARN_ON_ONCE(1);
>>> 		return 0;
>>> @@ -490,11 +498,11 @@ size_t nfs_generic_pg_test(struct nfs_pageio_descriptor *desc,
>>> 	 * Limit the request size so that we can still allocate a page array
>>> 	 * for it without upsetting the slab allocator.
>>> 	 */
>>> -	if (((desc->pg_count + req->wb_bytes) >> PAGE_SHIFT) *
>>> +	if (((mirror->pg_count + req->wb_bytes) >> PAGE_SHIFT) *
>>> 			sizeof(struct page) > PAGE_SIZE)
>>> 		return 0;
>>>
>>> -	return min(desc->pg_bsize - desc->pg_count, (size_t)req->wb_bytes);
>>> +	return min(mirror->pg_bsize - mirror->pg_count, (size_t)req->wb_bytes);
>>> }
>>> EXPORT_SYMBOL_GPL(nfs_generic_pg_test);
>>>
>>> @@ -651,10 +659,18 @@ EXPORT_SYMBOL_GPL(nfs_initiate_pgio);
>>> static int nfs_pgio_error(struct nfs_pageio_descriptor *desc,
>>> 			  struct nfs_pgio_header *hdr)
>>> {
>>> +	struct nfs_pgio_mirror *mirror;
>>> +	u32 midx;
>>> +
>>> 	set_bit(NFS_IOHDR_REDO, &hdr->flags);
>>> 	nfs_pgio_data_destroy(hdr);
>>> 	hdr->completion_ops->completion(hdr);
>>> -	desc->pg_completion_ops->error_cleanup(&desc->pg_list);
>>> +	/* TODO: Make sure it's right to clean up all mirrors here
>>> +	 *       and not just hdr->pgio_mirror_idx */
>>> +	for (midx = 0; midx < desc->pg_mirror_count; midx++) {
>>> +		mirror = &desc->pg_mirrors[midx];
>>> +		desc->pg_completion_ops->error_cleanup(&mirror->pg_list);
>>> +	}
>>> 	return -ENOMEM;
>>> }
>>>
>>> @@ -671,6 +687,17 @@ static void nfs_pgio_release(void *calldata)
>>> 	hdr->completion_ops->completion(hdr);
>>> }
>>>
>>> +static void nfs_pageio_mirror_init(struct nfs_pgio_mirror *mirror,
>>> +				   unsigned int bsize)
>>> +{
>>> +	INIT_LIST_HEAD(&mirror->pg_list);
>>> +	mirror->pg_bytes_written = 0;
>>> +	mirror->pg_count = 0;
>>> +	mirror->pg_bsize = bsize;
>>> +	mirror->pg_base = 0;
>>> +	mirror->pg_recoalesce = 0;
>>> +}
>>> +
>>> /**
>>>  * nfs_pageio_init - initialise a page io descriptor
>>>  * @desc: pointer to descriptor
>>> @@ -687,13 +714,10 @@ void nfs_pageio_init(struct nfs_pageio_descriptor *desc,
>>> 		     size_t bsize,
>>> 		     int io_flags)
>>> {
>>> -	INIT_LIST_HEAD(&desc->pg_list);
>>> -	desc->pg_bytes_written = 0;
>>> -	desc->pg_count = 0;
>>> -	desc->pg_bsize = bsize;
>>> -	desc->pg_base = 0;
>>> +	struct nfs_pgio_mirror *new;
>>> +	int i;
>>> +
>>> 	desc->pg_moreio = 0;
>>> -	desc->pg_recoalesce = 0;
>>> 	desc->pg_inode = inode;
>>> 	desc->pg_ops = pg_ops;
>>> 	desc->pg_completion_ops = compl_ops;
>>> @@ -703,6 +727,26 @@ void nfs_pageio_init(struct nfs_pageio_descriptor *desc,
>>> 	desc->pg_lseg = NULL;
>>> 	desc->pg_dreq = NULL;
>>> 	desc->pg_layout_private = NULL;
>>> +	desc->pg_bsize = bsize;
>>> +
>>> +	desc->pg_mirror_count = 1;
>>> +	desc->pg_mirror_idx = 0;
>>> +
>>> +	if (pg_ops->pg_get_mirror_count) {
>>> +		/* until we have a request, we don't have an lseg and no
>>> +		 * idea how many mirrors there will be */
>>> +		new = kcalloc(NFS_PAGEIO_DESCRIPTOR_MIRROR_MAX,
>>> +			      sizeof(struct nfs_pgio_mirror), GFP_KERNEL);
>>> +		desc->pg_mirrors_dynamic = new;
>>> +		desc->pg_mirrors = new;
>>> +
>>> +		for (i = 0; i < NFS_PAGEIO_DESCRIPTOR_MIRROR_MAX; i++)
>>> +			nfs_pageio_mirror_init(&desc->pg_mirrors[i], bsize);
>>> +	} else {
>>> +		desc->pg_mirrors_dynamic = NULL;
>>> +		desc->pg_mirrors = desc->pg_mirrors_static;
>>> +		nfs_pageio_mirror_init(&desc->pg_mirrors[0], bsize);
>>> +	}
>>> }
>>> EXPORT_SYMBOL_GPL(nfs_pageio_init);
>>>
>>> @@ -738,14 +782,16 @@ static void nfs_pgio_result(struct rpc_task *task, void *calldata)
>>> int nfs_generic_pgio(struct nfs_pageio_descriptor *desc,
>>> 		     struct nfs_pgio_header *hdr)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> 	struct nfs_page		*req;
>>> 	struct page		**pages,
>>> 				*last_page;
>>> -	struct list_head *head = &desc->pg_list;
>>> +	struct list_head *head = &mirror->pg_list;
>>> 	struct nfs_commit_info cinfo;
>>> 	unsigned int pagecount, pageused;
>>>
>>> -	pagecount = nfs_page_array_len(desc->pg_base, desc->pg_count);
>>> +	pagecount = nfs_page_array_len(mirror->pg_base, mirror->pg_count);
>>> 	if (!nfs_pgarray_set(&hdr->page_array, pagecount))
>>> 		return nfs_pgio_error(desc, hdr);
>>>
>>> @@ -773,7 +819,7 @@ int nfs_generic_pgio(struct nfs_pageio_descriptor *desc,
>>> 		desc->pg_ioflags &= ~FLUSH_COND_STABLE;
>>>
>>> 	/* Set up the argument struct */
>>> -	nfs_pgio_rpcsetup(hdr, desc->pg_count, 0, desc->pg_ioflags, &cinfo);
>>> +	nfs_pgio_rpcsetup(hdr, mirror->pg_count, 0, desc->pg_ioflags, &cinfo);
>>> 	desc->pg_rpc_callops = &nfs_pgio_common_ops;
>>> 	return 0;
>>> }
>>> @@ -781,12 +827,17 @@ EXPORT_SYMBOL_GPL(nfs_generic_pgio);
>>>
>>> static int nfs_generic_pg_pgios(struct nfs_pageio_descriptor *desc)
>>> {
>>> +	struct nfs_pgio_mirror *mirror;
>>> 	struct nfs_pgio_header *hdr;
>>> 	int ret;
>>>
>>> +	mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> 	hdr = nfs_pgio_header_alloc(desc->pg_rw_ops);
>>> 	if (!hdr) {
>>> -		desc->pg_completion_ops->error_cleanup(&desc->pg_list);
>>> +		/* TODO: make sure this is right with mirroring - or
>>> +		 *       should it back out all mirrors? */
>>> +		desc->pg_completion_ops->error_cleanup(&mirror->pg_list);
>>> 		return -ENOMEM;
>>> 	}
>>> 	nfs_pgheader_init(desc, hdr, nfs_pgio_header_free);
>>> @@ -801,6 +852,49 @@ static int nfs_generic_pg_pgios(struct nfs_pageio_descriptor *desc)
>>> 	return ret;
>>> }
>>>
>>> +/*
>>> + * nfs_pageio_setup_mirroring - determine if mirroring is to be used
>>> + *				by calling the pg_get_mirror_count op
>>> + */
>>> +static int nfs_pageio_setup_mirroring(struct nfs_pageio_descriptor *pgio,
>>> +				       struct nfs_page *req)
>>> +{
>>> +	int mirror_count = 1;
>>> +
>>> +	if (!pgio->pg_ops->pg_get_mirror_count)
>>> +		return 0;
>>> +
>>> +	mirror_count = pgio->pg_ops->pg_get_mirror_count(pgio, req);
>>> +
>>> +	if (!mirror_count || mirror_count > NFS_PAGEIO_DESCRIPTOR_MIRROR_MAX)
>>> +		return -EINVAL;
>>> +
>>> +	if (WARN_ON_ONCE(!pgio->pg_mirrors_dynamic))
>>> +		return -EINVAL;
>>> +
>>> +	pgio->pg_mirror_count = mirror_count;
>>> +
>>> +	return 0;
>>> +}
>>> +
>>> +/*
>>> + * nfs_pageio_stop_mirroring - stop using mirroring (set mirror count to 1)
>>> + */
>>> +void nfs_pageio_stop_mirroring(struct nfs_pageio_descriptor *pgio)
>>> +{
>>> +	pgio->pg_mirror_count = 1;
>>> +	pgio->pg_mirror_idx = 0;
>>> +}
>>> +
>>> +static void nfs_pageio_cleanup_mirroring(struct nfs_pageio_descriptor *pgio)
>>> +{
>>> +	pgio->pg_mirror_count = 1;
>>> +	pgio->pg_mirror_idx = 0;
>>> +	pgio->pg_mirrors = pgio->pg_mirrors_static;
>>> +	kfree(pgio->pg_mirrors_dynamic);
>>> +	pgio->pg_mirrors_dynamic = NULL;
>>> +}
>>> +
>>> static bool nfs_match_open_context(const struct nfs_open_context *ctx1,
>>> 		const struct nfs_open_context *ctx2)
>>> {
>>> @@ -867,19 +961,22 @@ static bool nfs_can_coalesce_requests(struct nfs_page *prev,
>>> static int nfs_pageio_do_add_request(struct nfs_pageio_descriptor *desc,
>>> 				     struct nfs_page *req)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> 	struct nfs_page *prev = NULL;
>>> -	if (desc->pg_count != 0) {
>>> -		prev = nfs_list_entry(desc->pg_list.prev);
>>> +
>>> +	if (mirror->pg_count != 0) {
>>> +		prev = nfs_list_entry(mirror->pg_list.prev);
>>> 	} else {
>>> 		if (desc->pg_ops->pg_init)
>>> 			desc->pg_ops->pg_init(desc, req);
>>> -		desc->pg_base = req->wb_pgbase;
>>> +		mirror->pg_base = req->wb_pgbase;
>>> 	}
>>> 	if (!nfs_can_coalesce_requests(prev, req, desc))
>>> 		return 0;
>>> 	nfs_list_remove_request(req);
>>> -	nfs_list_add_request(req, &desc->pg_list);
>>> -	desc->pg_count += req->wb_bytes;
>>> +	nfs_list_add_request(req, &mirror->pg_list);
>>> +	mirror->pg_count += req->wb_bytes;
>>> 	return 1;
>>> }
>>>
>>> @@ -888,16 +985,19 @@ static int nfs_pageio_do_add_request(struct nfs_pageio_descriptor *desc,
>>>  */
>>> static void nfs_pageio_doio(struct nfs_pageio_descriptor *desc)
>>> {
>>> -	if (!list_empty(&desc->pg_list)) {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> +
>>> +	if (!list_empty(&mirror->pg_list)) {
>>> 		int error = desc->pg_ops->pg_doio(desc);
>>> 		if (error < 0)
>>> 			desc->pg_error = error;
>>> 		else
>>> -			desc->pg_bytes_written += desc->pg_count;
>>> +			mirror->pg_bytes_written += mirror->pg_count;
>>> 	}
>>> -	if (list_empty(&desc->pg_list)) {
>>> -		desc->pg_count = 0;
>>> -		desc->pg_base = 0;
>>> +	if (list_empty(&mirror->pg_list)) {
>>> +		mirror->pg_count = 0;
>>> +		mirror->pg_base = 0;
>>> 	}
>>> }
>>>
>>> @@ -915,10 +1015,14 @@ static void nfs_pageio_doio(struct nfs_pageio_descriptor *desc)
>>> static int __nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
>>> 			   struct nfs_page *req)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> 	struct nfs_page *subreq;
>>> 	unsigned int bytes_left = 0;
>>> 	unsigned int offset, pgbase;
>>>
>>> +	WARN_ON_ONCE(desc->pg_mirror_idx >= desc->pg_mirror_count);
>>> +
>>> 	nfs_page_group_lock(req, false);
>>>
>>> 	subreq = req;
>>> @@ -938,7 +1042,7 @@ static int __nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
>>> 			nfs_pageio_doio(desc);
>>> 			if (desc->pg_error < 0)
>>> 				return 0;
>>> -			if (desc->pg_recoalesce)
>>> +			if (mirror->pg_recoalesce)
>>> 				return 0;
>>> 			/* retry add_request for this subreq */
>>> 			nfs_page_group_lock(req, false);
>>> @@ -976,14 +1080,16 @@ err_ptr:
>>>
>>> static int nfs_do_recoalesce(struct nfs_pageio_descriptor *desc)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> 	LIST_HEAD(head);
>>>
>>> 	do {
>>> -		list_splice_init(&desc->pg_list, &head);
>>> -		desc->pg_bytes_written -= desc->pg_count;
>>> -		desc->pg_count = 0;
>>> -		desc->pg_base = 0;
>>> -		desc->pg_recoalesce = 0;
>>> +		list_splice_init(&mirror->pg_list, &head);
>>> +		mirror->pg_bytes_written -= mirror->pg_count;
>>> +		mirror->pg_count = 0;
>>> +		mirror->pg_base = 0;
>>> +		mirror->pg_recoalesce = 0;
>>> +
>>> 		desc->pg_moreio = 0;
>>>
>>> 		while (!list_empty(&head)) {
>>> @@ -997,11 +1103,11 @@ static int nfs_do_recoalesce(struct nfs_pageio_descriptor *desc)
>>> 				return 0;
>>> 			break;
>>> 		}
>>> -	} while (desc->pg_recoalesce);
>>> +	} while (mirror->pg_recoalesce);
>>> 	return 1;
>>> }
>>>
>>> -int nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
>>> +static int nfs_pageio_add_request_mirror(struct nfs_pageio_descriptor *desc,
>>> 		struct nfs_page *req)
>>> {
>>> 	int ret;
>>> @@ -1014,9 +1120,78 @@ int nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
>>> 			break;
>>> 		ret = nfs_do_recoalesce(desc);
>>> 	} while (ret);
>>> +
>>> 	return ret;
>>> }
>>>
>>> +int nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
>>> +			   struct nfs_page *req)
>>> +{
>>> +	u32 midx;
>>> +	unsigned int pgbase, offset, bytes;
>>> +	struct nfs_page *dupreq, *lastreq;
>>> +
>>> +	pgbase = req->wb_pgbase;
>>> +	offset = req->wb_offset;
>>> +	bytes = req->wb_bytes;
>>> +
>>> +	nfs_pageio_setup_mirroring(desc, req);
>>> +
>>> +	for (midx = 0; midx < desc->pg_mirror_count; midx++) {
>>> +		if (midx) {
>>> +			nfs_page_group_lock(req, false);
>>> +
>>> +			/* find the last request */
>>> +			for (lastreq = req->wb_head;
>>> +			     lastreq->wb_this_page != req->wb_head;
>>> +			     lastreq = lastreq->wb_this_page)
>>> +				;
>>> +
>>> +			dupreq = nfs_create_request(req->wb_context,
>>> +					req->wb_page, lastreq, pgbase, bytes);
>>> +
>>> +			if (IS_ERR(dupreq)) {
>>> +				nfs_page_group_unlock(req);
>>> +				return 0;
>>> +			}
>>> +
>>> +			nfs_lock_request(dupreq);
>>> +			nfs_page_group_unlock(req);
>>> +			dupreq->wb_offset = offset;
>>> +			dupreq->wb_index = req->wb_index;
>>> +		} else
>>> +			dupreq = req;
>>> +
>>> +		desc->pg_mirror_idx = midx;
>>> +		if (!nfs_pageio_add_request_mirror(desc, dupreq))
>>> +			return 0;
>>> +	}
>>> +
>>> +	return 1;
>>> +}
>>> +
>>> +/*
>>> + * nfs_pageio_complete_mirror - Complete I/O on the current mirror of an
>>> + *				nfs_pageio_descriptor
>>> + * @desc: pointer to io descriptor
>>> + */
>>> +static void nfs_pageio_complete_mirror(struct nfs_pageio_descriptor *desc,
>>> +				       u32 mirror_idx)
>>> +{
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[mirror_idx];
>>> +	u32 restore_idx = desc->pg_mirror_idx;
>>> +
>>> +	desc->pg_mirror_idx = mirror_idx;
>>> +	for (;;) {
>>> +		nfs_pageio_doio(desc);
>>> +		if (!mirror->pg_recoalesce)
>>> +			break;
>>> +		if (!nfs_do_recoalesce(desc))
>>> +			break;
>>> +	}
>>> +	desc->pg_mirror_idx = restore_idx;
>>> +}
>>> +
>>> /*
>>>  * nfs_pageio_resend - Transfer requests to new descriptor and resend
>>>  * @hdr - the pgio header to move request from
>>> @@ -1055,16 +1230,14 @@ EXPORT_SYMBOL_GPL(nfs_pageio_resend);
>>>  */
>>> void nfs_pageio_complete(struct nfs_pageio_descriptor *desc)
>>> {
>>> -	for (;;) {
>>> -		nfs_pageio_doio(desc);
>>> -		if (!desc->pg_recoalesce)
>>> -			break;
>>> -		if (!nfs_do_recoalesce(desc))
>>> -			break;
>>> -	}
>>> +	u32 midx;
>>> +
>>> +	for (midx = 0; midx < desc->pg_mirror_count; midx++)
>>> +		nfs_pageio_complete_mirror(desc, midx);
>>>
>>> 	if (desc->pg_ops->pg_cleanup)
>>> 		desc->pg_ops->pg_cleanup(desc);
>>> +	nfs_pageio_cleanup_mirroring(desc);
>>> }
>>>
>>> /**
>>> @@ -1080,10 +1253,17 @@ void nfs_pageio_complete(struct nfs_pageio_descriptor *desc)
>>>  */
>>> void nfs_pageio_cond_complete(struct nfs_pageio_descriptor *desc, pgoff_t index)
>>> {
>>> -	if (!list_empty(&desc->pg_list)) {
>>> -		struct nfs_page *prev = nfs_list_entry(desc->pg_list.prev);
>>> -		if (index != prev->wb_index + 1)
>>> -			nfs_pageio_complete(desc);
>>> +	struct nfs_pgio_mirror *mirror;
>>> +	struct nfs_page *prev;
>>> +	u32 midx;
>>> +
>>> +	for (midx = 0; midx < desc->pg_mirror_count; midx++) {
>>> +		mirror = &desc->pg_mirrors[midx];
>>> +		if (!list_empty(&mirror->pg_list)) {
>>> +			prev = nfs_list_entry(mirror->pg_list.prev);
>>> +			if (index != prev->wb_index + 1)
>>> +				nfs_pageio_complete_mirror(desc, midx);
>>> +		}
>>> 	}
>>> }
>>>
>>> diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
>>> index 2da2e77..5f7c422 100644
>>> --- a/fs/nfs/pnfs.c
>>> +++ b/fs/nfs/pnfs.c
>>> @@ -1646,8 +1646,8 @@ EXPORT_SYMBOL_GPL(pnfs_generic_pg_cleanup);
>>>  * of bytes (maximum @req->wb_bytes) that can be coalesced.
>>>  */
>>> size_t
>>> -pnfs_generic_pg_test(struct nfs_pageio_descriptor *pgio, struct nfs_page *prev,
>>> -		     struct nfs_page *req)
>>> +pnfs_generic_pg_test(struct nfs_pageio_descriptor *pgio,
>>> +		     struct nfs_page *prev, struct nfs_page *req)
>>> {
>>> 	unsigned int size;
>>> 	u64 seg_end, req_start, seg_left;
>>> @@ -1729,10 +1729,12 @@ static void
>>> pnfs_write_through_mds(struct nfs_pageio_descriptor *desc,
>>> 		struct nfs_pgio_header *hdr)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> 	if (!test_and_set_bit(NFS_IOHDR_REDO, &hdr->flags)) {
>>> -		list_splice_tail_init(&hdr->pages, &desc->pg_list);
>>> +		list_splice_tail_init(&hdr->pages, &mirror->pg_list);
>>> 		nfs_pageio_reset_write_mds(desc);
>>> -		desc->pg_recoalesce = 1;
>>> +		mirror->pg_recoalesce = 1;
>>> 	}
>>> 	nfs_pgio_data_destroy(hdr);
>>> }
>>> @@ -1781,12 +1783,14 @@ EXPORT_SYMBOL_GPL(pnfs_writehdr_free);
>>> int
>>> pnfs_generic_pg_writepages(struct nfs_pageio_descriptor *desc)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> 	struct nfs_pgio_header *hdr;
>>> 	int ret;
>>>
>>> 	hdr = nfs_pgio_header_alloc(desc->pg_rw_ops);
>>> 	if (!hdr) {
>>> -		desc->pg_completion_ops->error_cleanup(&desc->pg_list);
>>> +		desc->pg_completion_ops->error_cleanup(&mirror->pg_list);
>>> 		return -ENOMEM;
>>> 	}
>>> 	nfs_pgheader_init(desc, hdr, pnfs_writehdr_free);
>>> @@ -1795,6 +1799,7 @@ pnfs_generic_pg_writepages(struct nfs_pageio_descriptor *desc)
>>> 	ret = nfs_generic_pgio(desc, hdr);
>>> 	if (!ret)
>>> 		pnfs_do_write(desc, hdr, desc->pg_ioflags);
>>> +
>>> 	return ret;
>>> }
>>> EXPORT_SYMBOL_GPL(pnfs_generic_pg_writepages);
>>> @@ -1839,10 +1844,13 @@ static void
>>> pnfs_read_through_mds(struct nfs_pageio_descriptor *desc,
>>> 		struct nfs_pgio_header *hdr)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> +
>>> 	if (!test_and_set_bit(NFS_IOHDR_REDO, &hdr->flags)) {
>>> -		list_splice_tail_init(&hdr->pages, &desc->pg_list);
>>> +		list_splice_tail_init(&hdr->pages, &mirror->pg_list);
>>> 		nfs_pageio_reset_read_mds(desc);
>>> -		desc->pg_recoalesce = 1;
>>> +		mirror->pg_recoalesce = 1;
>>> 	}
>>> 	nfs_pgio_data_destroy(hdr);
>>> }
>>> @@ -1893,12 +1901,14 @@ EXPORT_SYMBOL_GPL(pnfs_readhdr_free);
>>> int
>>> pnfs_generic_pg_readpages(struct nfs_pageio_descriptor *desc)
>>> {
>>> +	struct nfs_pgio_mirror *mirror = &desc->pg_mirrors[desc->pg_mirror_idx];
>>> +
>>> 	struct nfs_pgio_header *hdr;
>>> 	int ret;
>>>
>>> 	hdr = nfs_pgio_header_alloc(desc->pg_rw_ops);
>>> 	if (!hdr) {
>>> -		desc->pg_completion_ops->error_cleanup(&desc->pg_list);
>>> +		desc->pg_completion_ops->error_cleanup(&mirror->pg_list);
>>> 		return -ENOMEM;
>>> 	}
>>> 	nfs_pgheader_init(desc, hdr, pnfs_readhdr_free);
>>> diff --git a/fs/nfs/read.c b/fs/nfs/read.c
>>> index 092ab49..568ecf0 100644
>>> --- a/fs/nfs/read.c
>>> +++ b/fs/nfs/read.c
>>> @@ -70,8 +70,15 @@ EXPORT_SYMBOL_GPL(nfs_pageio_init_read);
>>>
>>> void nfs_pageio_reset_read_mds(struct nfs_pageio_descriptor *pgio)
>>> {
>>> +	struct nfs_pgio_mirror *mirror;
>>> +
>>> 	pgio->pg_ops = &nfs_pgio_rw_ops;
>>> -	pgio->pg_bsize = NFS_SERVER(pgio->pg_inode)->rsize;
>>> +
>>> +	/* read path should never have more than one mirror */
>>> +	WARN_ON_ONCE(pgio->pg_mirror_count != 1);
>>> +
>>> +	mirror = &pgio->pg_mirrors[0];
>>> +	mirror->pg_bsize = NFS_SERVER(pgio->pg_inode)->rsize;
>>> }
>>> EXPORT_SYMBOL_GPL(nfs_pageio_reset_read_mds);
>>>
>>> @@ -81,6 +88,7 @@ int nfs_readpage_async(struct nfs_open_context *ctx, struct inode *inode,
>>> 	struct nfs_page	*new;
>>> 	unsigned int len;
>>> 	struct nfs_pageio_descriptor pgio;
>>> +	struct nfs_pgio_mirror *pgm;
>>>
>>> 	len = nfs_page_length(page);
>>> 	if (len == 0)
>>> @@ -97,7 +105,13 @@ int nfs_readpage_async(struct nfs_open_context *ctx, struct inode *inode,
>>> 			     &nfs_async_read_completion_ops);
>>> 	nfs_pageio_add_request(&pgio, new);
>>> 	nfs_pageio_complete(&pgio);
>>> -	NFS_I(inode)->read_io += pgio.pg_bytes_written;
>>> +
>>> +	/* It doesn't make sense to do mirrored reads! */
>>> +	WARN_ON_ONCE(pgio.pg_mirror_count != 1);
>>> +
>>> +	pgm = &pgio.pg_mirrors[0];
>>> +	NFS_I(inode)->read_io += pgm->pg_bytes_written;
>>> +
>>> 	return 0;
>>> }
>>>
>>> @@ -352,6 +366,7 @@ int nfs_readpages(struct file *filp, struct address_space *mapping,
>>> 		struct list_head *pages, unsigned nr_pages)
>>> {
>>> 	struct nfs_pageio_descriptor pgio;
>>> +	struct nfs_pgio_mirror *pgm;
>>> 	struct nfs_readdesc desc = {
>>> 		.pgio = &pgio,
>>> 	};
>>> @@ -387,10 +402,15 @@ int nfs_readpages(struct file *filp, struct address_space *mapping,
>>> 			     &nfs_async_read_completion_ops);
>>>
>>> 	ret = read_cache_pages(mapping, pages, readpage_async_filler, &desc);
>>> -
>>> 	nfs_pageio_complete(&pgio);
>>> -	NFS_I(inode)->read_io += pgio.pg_bytes_written;
>>> -	npages = (pgio.pg_bytes_written + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
>>> +
>>> +	/* It doesn't make sense to do mirrored reads! */
>>> +	WARN_ON_ONCE(pgio.pg_mirror_count != 1);
>>> +
>>> +	pgm = &pgio.pg_mirrors[0];
>>> +	NFS_I(inode)->read_io += pgm->pg_bytes_written;
>>> +	npages = (pgm->pg_bytes_written + PAGE_CACHE_SIZE - 1) >>
>>> +		 PAGE_CACHE_SHIFT;
>>> 	nfs_add_stats(inode, NFSIOS_READPAGES, npages);
>>> read_complete:
>>> 	put_nfs_open_context(desc.ctx);
>>> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
>>> index db802d9..2f6ee8e 100644
>>> --- a/fs/nfs/write.c
>>> +++ b/fs/nfs/write.c
>>> @@ -906,7 +906,7 @@ static void nfs_write_completion(struct nfs_pgio_header *hdr)
>>> 		if (nfs_write_need_commit(hdr)) {
>>> 			memcpy(&req->wb_verf, &hdr->verf.verifier, sizeof(req->wb_verf));
>>> 			nfs_mark_request_commit(req, hdr->lseg, &cinfo,
>>> -				0);
>>> +				hdr->pgio_mirror_idx);
>>> 			goto next;
>>> 		}
>>> remove_req:
>>> @@ -1305,8 +1305,14 @@ EXPORT_SYMBOL_GPL(nfs_pageio_init_write);
>>>
>>> void nfs_pageio_reset_write_mds(struct nfs_pageio_descriptor *pgio)
>>> {
>>> +	struct nfs_pgio_mirror *mirror;
>>> +
>>> 	pgio->pg_ops = &nfs_pgio_rw_ops;
>>> -	pgio->pg_bsize = NFS_SERVER(pgio->pg_inode)->wsize;
>>> +
>>> +	nfs_pageio_stop_mirroring(pgio);
>>> +
>>> +	mirror = &pgio->pg_mirrors[0];
>>> +	mirror->pg_bsize = NFS_SERVER(pgio->pg_inode)->wsize;
>>> }
>>> EXPORT_SYMBOL_GPL(nfs_pageio_reset_write_mds);
>>>
>>> diff --git a/include/linux/nfs_page.h b/include/linux/nfs_page.h
>>> index 479c566..3eb072d 100644
>>> --- a/include/linux/nfs_page.h
>>> +++ b/include/linux/nfs_page.h
>>> @@ -58,6 +58,8 @@ struct nfs_pageio_ops {
>>> 	size_t	(*pg_test)(struct nfs_pageio_descriptor *, struct nfs_page *,
>>> 			   struct nfs_page *);
>>> 	int	(*pg_doio)(struct nfs_pageio_descriptor *);
>>> +	unsigned int	(*pg_get_mirror_count)(struct nfs_pageio_descriptor *,
>>> +				       struct nfs_page *);
>>> 	void	(*pg_cleanup)(struct nfs_pageio_descriptor *);
>>> };
>>>
>>> @@ -74,15 +76,17 @@ struct nfs_rw_ops {
>>> 			    struct rpc_task_setup *, int);
>>> };
>>>
>>> -struct nfs_pageio_descriptor {
>>> +struct nfs_pgio_mirror {
>>> 	struct list_head	pg_list;
>>> 	unsigned long		pg_bytes_written;
>>> 	size_t			pg_count;
>>> 	size_t			pg_bsize;
>>> 	unsigned int		pg_base;
>>> -	unsigned char		pg_moreio : 1,
>>> -				pg_recoalesce : 1;
>>> +	unsigned char		pg_recoalesce : 1;
>>> +};
>>>
>>> +struct nfs_pageio_descriptor {
>>> +	unsigned char		pg_moreio : 1;
>>> 	struct inode		*pg_inode;
>>> 	const struct nfs_pageio_ops *pg_ops;
>>> 	const struct nfs_rw_ops *pg_rw_ops;
>>> @@ -93,8 +97,18 @@ struct nfs_pageio_descriptor {
>>> 	struct pnfs_layout_segment *pg_lseg;
>>> 	struct nfs_direct_req	*pg_dreq;
>>> 	void			*pg_layout_private;
>>> +	unsigned int		pg_bsize;	/* default bsize for mirrors */
>>> +
>>> +	u32			pg_mirror_count;
>>> +	struct nfs_pgio_mirror	*pg_mirrors;
>>> +	struct nfs_pgio_mirror	pg_mirrors_static[1];
>>> +	struct nfs_pgio_mirror	*pg_mirrors_dynamic;
>>> +	u32			pg_mirror_idx;	/* current mirror */
>>> };
>>>
>>> +/* arbitrarily selected limit to number of mirrors */
>>> +#define NFS_PAGEIO_DESCRIPTOR_MIRROR_MAX 16
>>> +
>>> #define NFS_WBACK_BUSY(req)	(test_bit(PG_BUSY,&(req)->wb_flags))
>>>
>>> extern	struct nfs_page *nfs_create_request(struct nfs_open_context *ctx,
>>> diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
>>> index 5bc99f0..6400a1e 100644
>>> --- a/include/linux/nfs_xdr.h
>>> +++ b/include/linux/nfs_xdr.h
>>> @@ -1329,6 +1329,7 @@ struct nfs_pgio_header {
>>> 	struct nfs_page_array	page_array;
>>> 	struct nfs_client	*ds_clp;	/* pNFS data server */
>>> 	int			ds_commit_idx;	/* ds index if ds_clp is set */
>>> +	int			pgio_mirror_idx;/* mirror index in pgio layer */
>>> };
>>>
>>> struct nfs_mds_commit_info {
>>>
>>
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