Return-Path: linux-nfs-owner@vger.kernel.org Received: from mail-qc0-f172.google.com ([209.85.216.172]:33866 "EHLO mail-qc0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757906AbaDXQwv (ORCPT ); Thu, 24 Apr 2014 12:52:51 -0400 Received: by mail-qc0-f172.google.com with SMTP id i8so2764393qcq.17 for ; Thu, 24 Apr 2014 09:52:51 -0700 (PDT) Date: Thu, 24 Apr 2014 12:52:48 -0400 From: Jeff Layton To: Weston Andros Adamson Cc: Trond Myklebust , linux-nfs list Subject: Re: [PATCH 05/17] nfs: add support for multiple nfs reqs per page Message-ID: <20140424125248.64a12518@tlielax.poochiereds.net> In-Reply-To: References: <1398202165-78897-1-git-send-email-dros@primarydata.com> <1398202165-78897-6-git-send-email-dros@primarydata.com> <20140424105000.665c159e@tlielax.poochiereds.net> <3DEC13E0-CC9B-498D-A97A-98D77628F672@primarydata.com> <20140424114507.39b24196@tlielax.poochiereds.net> MIME-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Sender: linux-nfs-owner@vger.kernel.org List-ID: On Thu, 24 Apr 2014 12:15:08 -0400 Weston Andros Adamson wrote: > On Apr 24, 2014, at 11:45 AM, Jeff Layton wrote: > > > On Thu, 24 Apr 2014 11:23:19 -0400 > > Weston Andros Adamson wrote: > > > >> On Apr 24, 2014, at 10:50 AM, Jeff Layton wrote: > >> > >>> On Tue, 22 Apr 2014 17:29:13 -0400 > >>> Weston Andros Adamson 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 > >>>> --- > >>>> 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). > >> > >> Only in the buffered write case. Page->private is not set for read path / direct i/o path. > >> > >>> 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); > >> > >> Well, wb_page is a struct page and doesn?t have wb_this_page (which is in struct > >> nfs_page), but I see where you?re going with this. > >> > > > > Doh, right! Sorry, I threw that together in haste, but you get the > > idea. I was thinking you could go back to the page and dereference > > ->private. > > > >> A strategy like this only works if we always have page->private pointing to the head > >> request. We chose not to go that way because it messes with the buffered > >> write path?s setting / clearing of page private which interacts with the swappable > >> nfs pages code that everyone seems to be afraid to touch ;) > >> > >> So we decided to go this route (not messing with page_private) as a first step - we > >> certainly could add it later, but the current approach makes things less complex. > >> > > > > Ok, that makes sense. Thanks... > > > >>> > >>> ...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. > >> > >> So, I?m not against using list_head.. I didn?t go that route initially because I was: > >> > >> 1) following the buffer_head example, which rolls it?s own list > >> > > > > I wouldn't be surprised if the buffer_head code predates the standard > > linked-list macros, so that probably explains why they did it that way. > > The file locking code has a similar construct in inode->i_flock list. > > AFAIK the sub-page functionality was added somewhat recently. > > > > >> 2) trying to grow nfs_page as little as possible - but we might have room within > >> the allocator bucket it currently lives in? > >> > > > > nfs_page comes out of a dedicated slabcache, so that probably won't be the case. > > Ah, right! > > > > >> 3) not sure list_head is suitable for a circular list (I haven?t ever looked into it). > >> > >> and until we have a way to find the head request (via page private, etc) without > >> walking the circular list (chicken / egg problem needing to grab head lock before walking > >> list to find the head to lock it), we?ll still need the head pointer. > >> > >> Thoughts? > >> > >> -dros > >> > > > > If you can't rely on page->private pointing to the request, then that > > does make it tough to do what I was suggesting. struct list_head lists > > are doubly-linked and circular by nature, so that does seem to be a > > natural fit for what you're trying to do. > > Oh I see -- you?re totally right about list_head being circular, one just has > to call for_each on whatever head they wish to start from. > > > > > The only problem is that struct list_head is two pointers instead of > > one, so it's not going to be as space-efficient as what you're doing > > here. If that's a large concern then you may have no choice but to do > > this after all. > > Right. How much do we care about an extra pointer here? It seems to me > that we should try to keep it as small as possible - I know Trond has been unwilling > to add members to rpc_task (for example) unless absolutely necessary and there will > be at least one (if not more) nfs_page structures per rpc_task. > Well there are potentially a lot of these structs, so an extra pointer in each adds up. In fact, if only the head req is ever on the per-inode list, then I guess the wb_list is unused for sub requests, right? That might be an opportunity for space savings too -- you could union wb_head and wb_list, and use a wb_flag to indicate which is valid... > One immediate takeaway: I need to add much better comments about this. > > As far as eventually removing the wb_head pointer, it gets really ugly to do without > changing the buffered write path (and swappable page semantics) because page_group > operations happen *after* nfs_inode_remove_request() clears page_private (syncing the > destruction of the page group). This means that nfs_release_request and > nfs_unlock_and_release_request will both have to be passed a previously cached head > pointer. yuck. > Ahh right -- that is tricky then. I'd have to ponder that a bit more... -- Jeff Layton