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

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: Re: [PATCH v6 2/4] xfs: Introduce a new function to find the desired type of offset from page cache
From: Jie Liu <jeff.liu@xxxxxxxxxx>
Date: Fri, 10 Aug 2012 16:02:40 +0800
Cc: xfs@xxxxxxxxxxx
In-reply-to: <5023CC01.3060201@xxxxxxx>
References: <501B6B6F.2090101@xxxxxxxxxx> <5023CC01.3060201@xxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120714 Thunderbird/14.0
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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