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    Subject[RFC PATCH 30/76] vfs, fscache: Force ->write_inode() to occur if cookie pinned for writeback
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    Use an inode flag, I_PINNING_FSCACHE_WB, to indicate that a cookie is
    pinned in use by that inode for the purposes of writeback.

    Pinning is necessary because the in-use pin from the open file is released
    before the writeback takes place, but if the resources aren't pinned, the
    dirty data can't be written to the cache.

    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    fs/fs-writeback.c | 8 ++++++++
    fs/fscache/io.c | 38 ++++++++++++++++++++++++++++++++++++++
    include/linux/fs.h | 3 +++
    include/linux/fscache.h | 22 ++++++++++++++++++++++
    include/linux/writeback.h | 1 +
    5 files changed, 72 insertions(+)

    diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    index e6005c78bfa9..f7fb7fb37337 100644
    --- a/fs/fs-writeback.c
    +++ b/fs/fs-writeback.c
    @@ -1506,6 +1506,13 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)

    if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
    inode->i_state |= I_DIRTY_PAGES;
    + else if (unlikely(inode->i_state & I_PINNING_FSCACHE_WB)) {
    + if (!(inode->i_state & I_DIRTY_PAGES)) {
    + inode->i_state &= ~I_PINNING_FSCACHE_WB;
    + wbc->unpinned_fscache_wb = true;
    + dirty |= I_PINNING_FSCACHE_WB; /* Cause write_inode */
    + }
    + }

    spin_unlock(&inode->i_lock);

    @@ -1517,6 +1524,7 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
    if (ret == 0)
    ret = err;
    }
    + wbc->unpinned_fscache_wb = false;
    trace_writeback_single_inode(inode, wbc, nr_to_write);
    return ret;
    }
    diff --git a/fs/fscache/io.c b/fs/fscache/io.c
    index 7ad13900b281..87ffe84c9f27 100644
    --- a/fs/fscache/io.c
    +++ b/fs/fscache/io.c
    @@ -142,3 +142,41 @@ int __fscache_begin_operation(struct fscache_cookie *cookie,
    return -ENOBUFS;
    }
    EXPORT_SYMBOL(__fscache_begin_operation);
    +
    +/**
    + * fscache_set_page_dirty - Mark page dirty and pin a cache object for writeback
    + * @page: The page being dirtied
    + * @cookie: The cookie referring to the cache object
    + *
    + * Set the dirty flag on a page and pin an in-use cache object in memory when
    + * dirtying a page so that writeback can later write to it. This is intended
    + * to be called from the filesystem's ->set_page_dirty() method.
    + *
    + * Returns 1 if PG_dirty was set on the page, 0 otherwise.
    + */
    +int fscache_set_page_dirty(struct page *page, struct fscache_cookie *cookie)
    +{
    + struct inode *inode = page->mapping->host;
    + bool need_use = false;
    +
    + _enter("");
    +
    + if (!__set_page_dirty_nobuffers(page))
    + return 0;
    + if (!fscache_cookie_valid(cookie))
    + return 1;
    +
    + if (!(inode->i_state & I_PINNING_FSCACHE_WB)) {
    + spin_lock(&inode->i_lock);
    + if (!(inode->i_state & I_PINNING_FSCACHE_WB)) {
    + inode->i_state |= I_PINNING_FSCACHE_WB;
    + need_use = true;
    + }
    + spin_unlock(&inode->i_lock);
    +
    + if (need_use)
    + fscache_use_cookie(cookie, true);
    + }
    + return 1;
    +}
    +EXPORT_SYMBOL(fscache_set_page_dirty);
    diff --git a/include/linux/fs.h b/include/linux/fs.h
    index f6dad577ea07..fdbac1252381 100644
    --- a/include/linux/fs.h
    +++ b/include/linux/fs.h
    @@ -2151,6 +2151,8 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
    * Used to detect that mark_inode_dirty() should not move
    * inode between dirty lists.
    *
    + * I_PINNING_FSCACHE_WB Inode is pinning an fscache object for writeback.
    + *
    * Q: What is the difference between I_WILL_FREE and I_FREEING?
    */
    #define I_DIRTY_SYNC (1 << 0)
    @@ -2173,6 +2175,7 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
    #define I_CREATING (1 << 15)
    #define I_DONTCACHE (1 << 16)
    #define I_SYNC_QUEUED (1 << 17)
    +#define I_PINNING_FSCACHE_WB (1 << 18)

    #define I_DIRTY_INODE (I_DIRTY_SYNC | I_DIRTY_DATASYNC)
    #define I_DIRTY (I_DIRTY_INODE | I_DIRTY_PAGES)
    diff --git a/include/linux/fscache.h b/include/linux/fscache.h
    index 8d47c7a08456..d2fc98a5755a 100644
    --- a/include/linux/fscache.h
    +++ b/include/linux/fscache.h
    @@ -19,6 +19,7 @@
    #include <linux/pagemap.h>
    #include <linux/pagevec.h>
    #include <linux/list_bl.h>
    +#include <linux/writeback.h>

    #if defined(CONFIG_FSCACHE) || defined(CONFIG_FSCACHE_MODULE)
    #define __fscache_available (1)
    @@ -580,4 +581,25 @@ int fscache_write(struct fscache_op_resources *opr,
    return ops->write(opr, start_pos, iter, term_func, term_func_priv);
    }

    +#if __fscache_available
    +extern int fscache_set_page_dirty(struct page *page, struct fscache_cookie *cookie);
    +#else
    +#define fscache_set_page_dirty(PAGE, COOKIE) (__set_page_dirty_nobuffers((PAGE)))
    +#endif
    +
    +/**
    + * fscache_unpin_writeback - Unpin writeback resources
    + * @wbc: The writeback control
    + * @cookie: The cookie referring to the cache object
    + *
    + * Unpin the writeback resources pinned by fscache_set_page_dirty(). This is
    + * intended to be called by the netfs's ->write_inode() method.
    + */
    +static inline void fscache_unpin_writeback(struct writeback_control *wbc,
    + struct fscache_cookie *cookie)
    +{
    + if (wbc->unpinned_fscache_wb)
    + fscache_unuse_cookie(cookie, NULL, NULL);
    +}
    +
    #endif /* _LINUX_FSCACHE_H */
    diff --git a/include/linux/writeback.h b/include/linux/writeback.h
    index 8e5c5bb16e2d..5e7d1d38f261 100644
    --- a/include/linux/writeback.h
    +++ b/include/linux/writeback.h
    @@ -69,6 +69,7 @@ struct writeback_control {
    unsigned for_reclaim:1; /* Invoked from the page allocator */
    unsigned range_cyclic:1; /* range_start is cyclic */
    unsigned for_sync:1; /* sync(2) WB_SYNC_ALL writeback */
    + unsigned unpinned_fscache_wb:1; /* Cleared I_PINNING_FSCACHE_WB */

    /*
    * When writeback IOs are bounced through async layers, only the

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