Re: [PATCH 05/17] nfs: add support for multiple nfs reqs per page

[Jeff Layton <jlayton@poochiereds.net>]

On Tue, 22 Apr 2014 17:29:13 -0400
Weston Andros Adamson <dros@primarydata.com> wrote:

> Add "page groups" - a circular list of nfs requests (struct nfs_page)
> that all reference the same page. This gives nfs read and write paths
> the ability to account for sub-page regions independently.  This
> somewhat follows the design of struct buffer_head's sub-page
> accounting.
> 
> Only "head" requests are ever added/removed from the inode list in
> the buffered write path. "head" and "sub" requests are treated the
> same through the read path and the rest of the write/commit path.
> Requests are given an extra reference across the life of the list.
> 
> Page groups are never rejoined after being split. If the read/write
> request fails and the client falls back to another path (ie revert
> to MDS in PNFS case), the already split requests are pushed through
> the recoalescing code again, which may split them further and then
> coalesce them into properly sized requests on the wire. Fragmentation
> shouldn't be a problem with the current design, because we flush all
> requests in page group when a non-contiguous request is added, so
> the only time resplitting should occur is on a resend of a read or
> write.
> 
> This patch lays the groundwork for sub-page splitting, but does not
> actually do any splitting. For now all page groups have one request
> as pg_test functions don't yet split pages. There are several related
> patches that are needed support multiple requests per page group.
> 
> Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
> ---
>  fs/nfs/direct.c          |   7 +-
>  fs/nfs/pagelist.c        | 218 ++++++++++++++++++++++++++++++++++++++++++++---
>  fs/nfs/read.c            |   4 +-
>  fs/nfs/write.c           |  12 ++-
>  include/linux/nfs_page.h |  12 ++-
>  5 files changed, 231 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
> index a0c30c5..9d968ca 100644
> --- a/fs/nfs/direct.c
> +++ b/fs/nfs/direct.c
> @@ -380,7 +380,7 @@ static ssize_t nfs_direct_read_schedule_segment(struct nfs_pageio_descriptor *de
>  			struct nfs_page *req;
>  			unsigned int req_len = min_t(size_t, bytes, PAGE_SIZE - pgbase);
>  			/* XXX do we need to do the eof zeroing found in async_filler? */
> -			req = nfs_create_request(dreq->ctx, pagevec[i],
> +			req = nfs_create_request(dreq->ctx, pagevec[i], NULL,
>  						 pgbase, req_len);
>  			if (IS_ERR(req)) {
>  				result = PTR_ERR(req);
> @@ -749,7 +749,7 @@ static ssize_t nfs_direct_write_schedule_segment(struct nfs_pageio_descriptor *d
>  			struct nfs_page *req;
>  			unsigned int req_len = min_t(size_t, bytes, PAGE_SIZE - pgbase);
>  
> -			req = nfs_create_request(dreq->ctx, pagevec[i],
> +			req = nfs_create_request(dreq->ctx, pagevec[i], NULL,
>  						 pgbase, req_len);
>  			if (IS_ERR(req)) {
>  				result = PTR_ERR(req);
> @@ -827,6 +827,8 @@ static void nfs_direct_write_completion(struct nfs_pgio_header *hdr)
>  	spin_unlock(&dreq->lock);
>  
>  	while (!list_empty(&hdr->pages)) {
> +		bool do_destroy = true;
> +
>  		req = nfs_list_entry(hdr->pages.next);
>  		nfs_list_remove_request(req);
>  		switch (bit) {
> @@ -834,6 +836,7 @@ static void nfs_direct_write_completion(struct nfs_pgio_header *hdr)
>  		case NFS_IOHDR_NEED_COMMIT:
>  			kref_get(&req->wb_kref);
>  			nfs_mark_request_commit(req, hdr->lseg, &cinfo);
> +			do_destroy = false;
>  		}
>  		nfs_unlock_and_release_request(req);
>  	}
> diff --git a/fs/nfs/pagelist.c b/fs/nfs/pagelist.c
> index ac4fb64..8cb8e14 100644
> --- a/fs/nfs/pagelist.c
> +++ b/fs/nfs/pagelist.c
> @@ -26,6 +26,8 @@
>  
>  static struct kmem_cache *nfs_page_cachep;
>  
> +static void nfs_free_request(struct nfs_page *);
> +
>  bool nfs_pgarray_set(struct nfs_page_array *p, unsigned int pagecount)
>  {
>  	p->npages = pagecount;
> @@ -133,10 +135,145 @@ nfs_iocounter_wait(struct nfs_io_counter *c)
>  	return __nfs_iocounter_wait(c);
>  }
>  
> +/*
> + * nfs_page_group_lock - lock the head of the page group
> + * @req - request in group that is to be locked
> + *
> + * this lock must be held if modifying the page group list
> + */
> +void
> +nfs_page_group_lock(struct nfs_page *req)
> +{
> +	struct nfs_page *head = req->wb_head;
> +	int err = -EAGAIN;
> +
> +	WARN_ON_ONCE(head != head->wb_head);
> +
> +	while (err)
> +		err = wait_on_bit_lock(&head->wb_flags, PG_HEADLOCK,
> +			nfs_wait_bit_killable, TASK_KILLABLE);
> +}
> +
> +/*
> + * nfs_page_group_unlock - unlock the head of the page group
> + * @req - request in group that is to be unlocked
> + */
> +void
> +nfs_page_group_unlock(struct nfs_page *req)
> +{
> +	struct nfs_page *head = req->wb_head;
> +
> +	WARN_ON_ONCE(head != head->wb_head);
> +
> +	smp_mb__before_clear_bit();
> +	clear_bit(PG_HEADLOCK, &head->wb_flags);
> +	smp_mb__after_clear_bit();
> +	wake_up_bit(&head->wb_flags, PG_HEADLOCK);
> +}
> +
> +/*
> + * nfs_page_group_sync_on_bit_locked
> + *
> + * must be called with page group lock held
> + */
> +static bool
> +nfs_page_group_sync_on_bit_locked(struct nfs_page *req, unsigned int bit)
> +{
> +	struct nfs_page *head = req->wb_head;
> +	struct nfs_page *tmp;
> +
> +	WARN_ON_ONCE(!test_bit(PG_HEADLOCK, &head->wb_flags));
> +	WARN_ON_ONCE(test_and_set_bit(bit, &req->wb_flags));
> +
> +	tmp = req->wb_this_page;
> +	while (tmp != req) {
> +		if (!test_bit(bit, &tmp->wb_flags))
> +			return false;
> +		tmp = tmp->wb_this_page;
> +	}
> +
> +	/* true! reset all bits */
> +	tmp = req;
> +	do {
> +		clear_bit(bit, &tmp->wb_flags);
> +		tmp = tmp->wb_this_page;
> +	} while (tmp != req);
> +
> +	return true;
> +}
> +
> +/*
> + * nfs_page_group_sync_on_bit - set bit on current request, but only
> + *   return true if the bit is set for all requests in page group
> + * @req - request in page group
> + * @bit - PG_* bit that is used to sync page group
> + */
> +bool nfs_page_group_sync_on_bit(struct nfs_page *req, unsigned int bit)
> +{
> +	bool ret;
> +
> +	nfs_page_group_lock(req);
> +	ret = nfs_page_group_sync_on_bit_locked(req, bit);
> +	nfs_page_group_unlock(req);
> +
> +	return ret;
> +}
> +
> +/*
> + * nfs_page_group_init - Initialize the page group linkage for @req
> + * @req - a new nfs request
> + * @prev - the previous request in page group, or NULL if @req is the first
> + *         or only request in the group (the head).
> + */
> +static inline void
> +nfs_page_group_init(struct nfs_page *req, struct nfs_page *prev)
> +{
> +	WARN_ON_ONCE(prev == req);
> +
> +	if (!prev) {
> +		req->wb_head = req;
> +		req->wb_this_page = req;
> +	} else {
> +		WARN_ON_ONCE(prev->wb_this_page != prev->wb_head);
> +		WARN_ON_ONCE(!test_bit(PG_HEADLOCK, &prev->wb_head->wb_flags));
> +		req->wb_head = prev->wb_head;
> +		req->wb_this_page = prev->wb_this_page;
> +		prev->wb_this_page = req;
> +	}
> +}
> +
> +/*
> + * nfs_page_group_destroy - sync the destruction of page groups
> + * @req - request that no longer needs the page group
> + *
> + * releases the page group reference from each member once all
> + * members have called this function.
> + */
> +static void
> +nfs_page_group_destroy(struct kref *kref)
> +{
> +	struct nfs_page *req = container_of(kref, struct nfs_page, wb_kref);
> +	struct nfs_page *tmp, *next;
> +
> +	if (!nfs_page_group_sync_on_bit(req, PG_TEARDOWN))
> +		return;
> +
> +	tmp = req;
> +	do {
> +		next = tmp->wb_this_page;
> +		/* unlink and free */
> +		tmp->wb_this_page = tmp;
> +		tmp->wb_head = tmp;
> +		nfs_free_request(tmp);
> +		tmp = next;
> +	} while (tmp != req);
> +}
> +
>  /**
>   * nfs_create_request - Create an NFS read/write request.
>   * @ctx: open context to use
>   * @page: page to write
> + * @last: last nfs request created for this page group or NULL if head
>   * @offset: starting offset within the page for the write
>   * @count: number of bytes to read/write
>   *
> @@ -146,7 +283,8 @@ nfs_iocounter_wait(struct nfs_io_counter *c)
>   */
>  struct nfs_page *
>  nfs_create_request(struct nfs_open_context *ctx, struct page *page,
> -		   unsigned int offset, unsigned int count)
> +		   struct nfs_page *last, unsigned int offset,
> +		   unsigned int count)
>  {
>  	struct nfs_page		*req;
>  	struct nfs_lock_context *l_ctx;
> @@ -178,6 +316,7 @@ nfs_create_request(struct nfs_open_context *ctx, struct page *page,
>  	req->wb_bytes   = count;
>  	req->wb_context = get_nfs_open_context(ctx);
>  	kref_init(&req->wb_kref);
> +	nfs_page_group_init(req, last);
>  	return req;
>  }
>  
> @@ -235,16 +374,22 @@ static void nfs_clear_request(struct nfs_page *req)
>  	}
>  }
>  
> -
>  /**
>   * nfs_release_request - Release the count on an NFS read/write request
>   * @req: request to release
>   *
>   * Note: Should never be called with the spinlock held!
>   */
> -static void nfs_free_request(struct kref *kref)
> +static void nfs_free_request(struct nfs_page *req)
>  {
> -	struct nfs_page *req = container_of(kref, struct nfs_page, wb_kref);
> +	WARN_ON_ONCE(req->wb_this_page != req);
> +
> +	/* extra debug: make sure no sync bits are still set */
> +	WARN_ON_ONCE(test_bit(PG_TEARDOWN, &req->wb_flags));
> +	WARN_ON_ONCE(test_bit(PG_UNLOCKPAGE, &req->wb_flags));
> +	WARN_ON_ONCE(test_bit(PG_UPTODATE, &req->wb_flags));
> +	WARN_ON_ONCE(test_bit(PG_WB_END, &req->wb_flags));
> +	WARN_ON_ONCE(test_bit(PG_REMOVE, &req->wb_flags));
>  
>  	/* Release struct file and open context */
>  	nfs_clear_request(req);
> @@ -253,7 +398,7 @@ static void nfs_free_request(struct kref *kref)
>  
>  void nfs_release_request(struct nfs_page *req)
>  {
> -	kref_put(&req->wb_kref, nfs_free_request);
> +	kref_put(&req->wb_kref, nfs_page_group_destroy);
>  }
>  
>  static int nfs_wait_bit_uninterruptible(void *word)
> @@ -439,21 +584,66 @@ static void nfs_pageio_doio(struct nfs_pageio_descriptor *desc)
>   * @desc: destination io descriptor
>   * @req: request
>   *
> + * This may split a request into subrequests which are all part of the
> + * same page group.
> + *
>   * Returns true if the request 'req' was successfully coalesced into the
>   * existing list of pages 'desc'.
>   */
>  static int __nfs_pageio_add_request(struct nfs_pageio_descriptor *desc,
>  			   struct nfs_page *req)
>  {
> -	while (!nfs_pageio_do_add_request(desc, req)) {
> -		desc->pg_moreio = 1;
> -		nfs_pageio_doio(desc);
> -		if (desc->pg_error < 0)
> -			return 0;
> -		desc->pg_moreio = 0;
> -		if (desc->pg_recoalesce)
> -			return 0;
> -	}
> +	struct nfs_page *subreq;
> +	unsigned int bytes_left = 0;
> +	unsigned int offset, pgbase;
> +
> +	nfs_page_group_lock(req);
> +
> +	subreq = req;
> +	bytes_left = subreq->wb_bytes;
> +	offset = subreq->wb_offset;
> +	pgbase = subreq->wb_pgbase;
> +
> +	do {
> +		if (!nfs_pageio_do_add_request(desc, subreq)) {
> +			/* make sure pg_test call(s) did nothing */
> +			WARN_ON_ONCE(subreq->wb_bytes != bytes_left);
> +			WARN_ON_ONCE(subreq->wb_offset != offset);
> +			WARN_ON_ONCE(subreq->wb_pgbase != pgbase);
> +
> +			nfs_page_group_unlock(req);
> +			desc->pg_moreio = 1;
> +			nfs_pageio_doio(desc);
> +			if (desc->pg_error < 0)
> +				return 0;
> +			desc->pg_moreio = 0;
> +			if (desc->pg_recoalesce)
> +				return 0;
> +			/* retry add_request for this subreq */
> +			nfs_page_group_lock(req);
> +			continue;
> +		}
> +
> +		/* check for buggy pg_test call(s) */
> +		WARN_ON_ONCE(subreq->wb_bytes + subreq->wb_pgbase > PAGE_SIZE);
> +		WARN_ON_ONCE(subreq->wb_bytes > bytes_left);
> +		WARN_ON_ONCE(subreq->wb_bytes == 0);
> +
> +		bytes_left -= subreq->wb_bytes;
> +		offset += subreq->wb_bytes;
> +		pgbase += subreq->wb_bytes;
> +
> +		if (bytes_left) {
> +			subreq = nfs_create_request(req->wb_context,
> +					req->wb_page,
> +					subreq, pgbase, bytes_left);
> +			nfs_lock_request(subreq);
> +			subreq->wb_offset  = offset;
> +			subreq->wb_index = req->wb_index;
> +		}
> +	} while (bytes_left > 0);
> +
> +	nfs_page_group_unlock(req);
>  	return 1;
>  }
>  
> diff --git a/fs/nfs/read.c b/fs/nfs/read.c
> index 95a0855..ee0a3cd 100644
> --- a/fs/nfs/read.c
> +++ b/fs/nfs/read.c
> @@ -139,7 +139,7 @@ int nfs_readpage_async(struct nfs_open_context *ctx, struct inode *inode,
>  	len = nfs_page_length(page);
>  	if (len == 0)
>  		return nfs_return_empty_page(page);
> -	new = nfs_create_request(ctx, page, 0, len);
> +	new = nfs_create_request(ctx, page, NULL, 0, len);
>  	if (IS_ERR(new)) {
>  		unlock_page(page);
>  		return PTR_ERR(new);
> @@ -600,7 +600,7 @@ readpage_async_filler(void *data, struct page *page)
>  	if (len == 0)
>  		return nfs_return_empty_page(page);
>  
> -	new = nfs_create_request(desc->ctx, page, 0, len);
> +	new = nfs_create_request(desc->ctx, page, NULL, 0, len);
>  	if (IS_ERR(new))
>  		goto out_error;
>  
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index ca20ec7..d1453f2 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -461,7 +461,7 @@ static void nfs_inode_remove_request(struct nfs_page *req)
>  	}
>  	nfsi->npages--;
>  	spin_unlock(&inode->i_lock);
> -	nfs_release_request(req);
> +	nfs_release_request(head);
>  }
>  
>  static void
> @@ -625,6 +625,7 @@ static void nfs_write_completion(struct nfs_pgio_header *hdr)
>  {
>  	struct nfs_commit_info cinfo;
>  	unsigned long bytes = 0;
> +	bool do_destroy;
>  
>  	if (test_bit(NFS_IOHDR_REDO, &hdr->flags))
>  		goto out;
> @@ -654,6 +655,7 @@ remove_req:
>  next:
>  		nfs_unlock_request(req);
>  		nfs_end_page_writeback(req->wb_page);
> +		do_destroy = !test_bit(NFS_IOHDR_NEED_COMMIT, &hdr->flags);
>  		nfs_release_request(req);
>  	}
>  out:
> @@ -758,6 +760,10 @@ static struct nfs_page *nfs_try_to_update_request(struct inode *inode,
>  		if (req == NULL)
>  			goto out_unlock;
>  
> +		/* should be handled by nfs_flush_incompatible */
> +		WARN_ON_ONCE(req->wb_head != req);
> +		WARN_ON_ONCE(req->wb_this_page != req);
> +
>  		rqend = req->wb_offset + req->wb_bytes;
>  		/*
>  		 * Tell the caller to flush out the request if
> @@ -819,7 +825,7 @@ static struct nfs_page * nfs_setup_write_request(struct nfs_open_context* ctx,
>  	req = nfs_try_to_update_request(inode, page, offset, bytes);
>  	if (req != NULL)
>  		goto out;
> -	req = nfs_create_request(ctx, page, offset, bytes);
> +	req = nfs_create_request(ctx, page, NULL, offset, bytes);
>  	if (IS_ERR(req))
>  		goto out;
>  	nfs_inode_add_request(inode, req);
> @@ -863,6 +869,8 @@ int nfs_flush_incompatible(struct file *file, struct page *page)
>  			return 0;
>  		l_ctx = req->wb_lock_context;
>  		do_flush = req->wb_page != page || req->wb_context != ctx;
> +		/* for now, flush if more than 1 request in page_group */
> +		do_flush |= req->wb_this_page != req;
>  		if (l_ctx && ctx->dentry->d_inode->i_flock != NULL) {
>  			do_flush |= l_ctx->lockowner.l_owner != current->files
>  				|| l_ctx->lockowner.l_pid != current->tgid;
> diff --git a/include/linux/nfs_page.h b/include/linux/nfs_page.h
> index 214e098..1fb161b 100644
> --- a/include/linux/nfs_page.h
> +++ b/include/linux/nfs_page.h
> @@ -26,6 +26,8 @@ enum {
>  	PG_MAPPED,		/* page private set for buffered io */
>  	PG_CLEAN,		/* write succeeded */
>  	PG_COMMIT_TO_DS,	/* used by pnfs layouts */
> +	PG_HEADLOCK,		/* page group lock of wb_head */
> +	PG_TEARDOWN,		/* page group sync for destroy */
>  };
>  
>  struct nfs_inode;
> @@ -41,6 +43,8 @@ struct nfs_page {
>  	struct kref		wb_kref;	/* reference count */
>  	unsigned long		wb_flags;
>  	struct nfs_write_verifier	wb_verf;	/* Commit cookie */
> +	struct nfs_page		*wb_this_page;  /* list of reqs for this page */
> +	struct nfs_page		*wb_head;       /* head pointer for req list */

Hmm ok, so to make sure I understand...

So page->private will point to the "head" req (struct page_private).
Then we'll have a singly-linked list of reqs hanging off of
wb_this_page. Is that right?

If so, then it seems like it would be clearer to use a standard
list_head here. If you need to get to the wb_head, you could always do
something like this:

	list_first_entry(&req->wb_page->wb_this_page);
 
...and could even turn that into a macro or static inline for some
syntactic sugar. It's a little more pointer chasing to find the head,
but it seems like that would be clearer than using yet another
linked-list implementation.

>  };
>  
>  struct nfs_pageio_descriptor;
> @@ -75,9 +79,10 @@ struct nfs_pageio_descriptor {
>  
>  extern	struct nfs_page *nfs_create_request(struct nfs_open_context *ctx,
>  					    struct page *page,
> +					    struct nfs_page *last,
>  					    unsigned int offset,
>  					    unsigned int count);
> -extern	void nfs_release_request(struct nfs_page *req);
> +extern	void nfs_release_request(struct nfs_page *);
>  
>  
>  extern	void nfs_pageio_init(struct nfs_pageio_descriptor *desc,
> @@ -95,7 +100,10 @@ extern size_t nfs_generic_pg_test(struct nfs_pageio_descriptor *desc,
>  				struct nfs_page *req);
>  extern  int nfs_wait_on_request(struct nfs_page *);
>  extern	void nfs_unlock_request(struct nfs_page *req);
> -extern	void nfs_unlock_and_release_request(struct nfs_page *req);
> +extern	void nfs_unlock_and_release_request(struct nfs_page *);
> +extern void nfs_page_group_lock(struct nfs_page *);
> +extern void nfs_page_group_unlock(struct nfs_page *);
> +extern bool nfs_page_group_sync_on_bit(struct nfs_page *, unsigned int);
>  
>  /*
>   * Lock the page of an asynchronous request


-- 
Jeff Layton <jlayton@poochiereds.net>