On 08/09/12 22:41, Mark Tinguely wrote:
> 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.
It's better to show me your code, this really optimize the logic a bit
than me.
>
>
>> +
>> + 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.
This is a fuzzy point to me. If there is no buffer heads associated
with a page, can we treat it as an invalid page like the mapping check
up above?
Thanks,
-Jeff
>
>> +
>> + 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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