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Re: [PATCH v6 2/4] xfs: Introduce a new function to find the desired typ

To: jeff.liu@xxxxxxxxxx
Subject: Re: [PATCH v6 2/4] xfs: Introduce a new function to find the desired type of offset from page cache
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Thu, 09 Aug 2012 09:41:05 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <501B6B6F.2090101@xxxxxxxxxx>
References: <501B6B6F.2090101@xxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:9.0) Gecko/20120122 Thunderbird/9.0
On 08/03/12 01:10, Jeff Liu wrote:
Introduce helpers to probe data or hole offset from page cache for unwritten 
extents.

---
  fs/xfs/xfs_file.c |  213 +++++++++++++++++++++++++++++++++++++++++++++++++++++
  1 files changed, 213 insertions(+), 0 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 98642cf..aff0c30 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -36,6 +36,7 @@



+ * keep the offset argument unchanged.
+ */
+STATIC bool
+xfs_lookup_buffer_offset(

I am fine with this helper routine.


+
+/*
+ * This routine is called to find out and return a data or hole offset
+ * from the page cache for unwritten extents according to the desired
+ * type for xfs_seek_data() or xfs_seek_hole().
+ *
+ * The argument offset is used to tell where we start to search from the
+ * page cache.  Map is used to figure out the end points of the range to
+ * lookup pages.
+ *
+ * Return true if the desired type of offset was found, and the argument
+ * offset is filled with that address.  Otherwise, return false and keep
+ * offset unchanged.
+ */
+STATIC bool
+xfs_find_get_desired_pgoff(
+       struct inode            *inode,
+       struct xfs_bmbt_irec    *map,
+       unsigned int            type,
+       loff_t                  *offset)
+{
+       struct xfs_inode        *ip = XFS_I(inode);
+       struct xfs_mount        *mp = ip->i_mount;
+       struct pagevec          pvec;
+       pgoff_t                 index;
+       pgoff_t                 end;
+       loff_t                  endoff;
+       loff_t                  startoff = *offset;
+       loff_t                  lastoff = startoff;
+       bool                    found = false;
+
+       pagevec_init(&pvec, 0);
+
+       index = startoff>>  PAGE_CACHE_SHIFT;
+       endoff = XFS_FSB_TO_B(mp, map->br_startoff + map->br_blockcount);
+       end = endoff>>  PAGE_CACHE_SHIFT;
+       do {
+               int             want;
+               unsigned        nr_pages;
+               unsigned int    i;
+
+               want = min_t(pgoff_t, end - index, (pgoff_t)PAGEVEC_SIZE);
+               nr_pages = pagevec_lookup(&pvec, inode->i_mapping, index,
+                                         want);
+               /*
+                * No page mapped into given range.  If we are searching holes
+                * and if this is the first time we got into the loop, it means
+                * that the given offset is landed in a hole, return it.
+                *
+                * If we have already stepped through some block buffers to find
+                * holes but they all contains data.  In this case, the last
+                * offset is already updated and pointed to the end of the last
+                * mapped page, if it does not reach the endpoint to search,
+                * that means there should be a hole between them.
+                */
+               if (nr_pages == 0) {
+                       /* Data search found nothing */
+                       if (type == DATA_OFF)
+                               break;
+
+                       ASSERT(type == HOLE_OFF);
+                       if (lastoff == startoff || lastoff<  endoff) {
+                               found = true;
+                               *offset = lastoff;
+                       }
+                       break;
+               }
+
+               /*
+                * At lease we found one page.  If this is the first time we
+                * step into the loop, and if the first page index offset is
+                * greater than the given search offset, a hole was found.
+                */
+               if (type == HOLE_OFF&&  lastoff == startoff&&
+                   lastoff<  page_offset(pvec.pages[0])) {
+                       found = true;
+                       break;
+               }

I played with the code and appreciate the trickiness of the startoff.
I this can be refined a bit more. see below.

+
+               for (i = 0; i<  nr_pages; i++) {
+                       struct page     *page = pvec.pages[i];
+                       loff_t          b_offset;
+
+                       /*
+                        * Page index is out of range, searching done.
+                        * If the current offset is not reaches the end
+                        * of the specified search range, there should
+                        * be a hole between them.
+                        */
+                       if (page->index>  end) {
+                               if (type == HOLE_OFF&&  lastoff<  endoff) {
+                                       *offset = lastoff;
+                                       found = true;
+                               }
+                               goto out;
+                       }

Before we take the lock, we can check to see if the page starts later than expected (a hole). The pre-loop start check can be removed and something like the follow added:

                        /* Skip over this page if it precedes the start offset 
*/
                        if (page_offset(page) + PAGE_SIZE < startoff)
                                continue;

if (type == HOLE_OFF && page_offset(page) > lastoff) {
                                *offset = lastoff;
                                found = true;
                                goto out;
                        }

My experiment passes your newest, more extensive tests. I can give you the
complete experiment patch if you want.


+
+                       lock_page(page);
+                       /*
+                        * Page truncated or invalidated(page->mapping == NULL).
+                        * We can freely skip it and proceed to check the next
+                        * page.
+                        */
+                       if (unlikely(page->mapping != inode->i_mapping)) {
+                               unlock_page(page);
+                               continue;
+                       }
+
+                       if (!page_has_buffers(page)) {
+                               unlock_page(page);
+                               continue;
+                       }

Would this be a hole condition? I suppose the next iteration of the loop will figure this out.

+
+                       found = xfs_lookup_buffer_offset(page,&b_offset, type);
+                       if (found) {
+                               /*
+                                * The found offset may be less than the start
+                                * point to search if this is the first time to
+                                * come here.
+                                */
+                               *offset = max_t(loff_t, startoff, b_offset);
+                               goto out;
+                       }
+
+                       /*
+                        * We either searching data but nothing was found, or
+                        * searching hole but found a data buffer.  In either
+                        * case, probably the next page contains the desired
+                        * things, update the last offset to it so.
+                        */
+                       lastoff = page_offset(page) + PAGE_SIZE;
+                       unlock_page(page);
+               }
+
+               /*
+                * The number of returned pages less than our desired, search
+                * done.  In this case, nothing was found for searching data,
+                * but we found a hole behind the last offset.
+                */
+               if (nr_pages<  want) {
+                       if (type == HOLE_OFF) {
+                               *offset = lastoff;
+                               found = true;
+                       }
+                       break;
+               }
+
+               index = pvec.pages[i - 1]->index + 1;
+               pagevec_release(&pvec);
+       } while (index<  end);
+
+out:
+       pagevec_release(&pvec);
+       return found;
+}
+
  STATIC loff_t
  xfs_seek_data(
        struct file             *file,

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