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    SubjectRe: O_DIRECT question
    On 1/12/07, Linus Torvalds <torvalds@osdl.org> wrote:
    >
    >
    > On Thu, 11 Jan 2007, Roy Huang wrote:
    > >
    > > On a embedded systerm, limiting page cache can relieve memory
    > > fragmentation. There is a patch against 2.6.19, which limit every
    > > opened file page cache and total pagecache. When the limit reach, it
    > > will release the page cache overrun the limit.
    >
    > I do think that something like this is probably a good idea, even on
    > non-embedded setups. We historically couldn't do this, because mapped
    > pages were too damn hard to remove, but that's obviously not much of a
    > problem any more.
    >
    > However, the page-cache limit should NOT be some compile-time constant. It
    > should work the same way the "dirty page" limit works, and probably just
    > default to "feel free to use 90% of memory for page cache".
    >
    > Linus
    >

    The attached patch limit the page cache by a simple way:

    1) If request memory from page cache, Set a flag to mark this kind of
    allocation:

    static inline struct page *page_cache_alloc(struct address_space *x)
    {
    - return __page_cache_alloc(mapping_gfp_mask(x));
    + return __page_cache_alloc(mapping_gfp_mask(x)|__GFP_PAGECACHE);
    }

    2) Have zone_watermark_ok done this limit:

    + if (alloc_flags & ALLOC_PAGECACHE){
    + min = min + VFS_CACHE_LIMIT;
    + }
    +
    if (free_pages <= min + z->lowmem_reserve[classzone_idx])
    return 0;

    3) So, when __alloc_pages is called by page cache, pass the
    ALLOC_PAGECACHE into get_page_from_freelist to trigger the pagecache
    limit branch in zone_watermark_ok.

    This approach works on my side, I'll make a new patch to make the
    limit tunable in the proc fs soon.

    The following is the patch:
    =====================================================
    Index: mm/page_alloc.c
    ===================================================================
    --- mm/page_alloc.c (revision 2645)
    +++ mm/page_alloc.c (working copy)
    @@ -892,6 +892,9 @@ failed:
    #define ALLOC_HARDER 0x10 /* try to alloc harder */
    #define ALLOC_HIGH 0x20 /* __GFP_HIGH set */
    #define ALLOC_CPUSET 0x40 /* check for correct cpuset */
    +#define ALLOC_PAGECACHE 0x80 /* __GFP_PAGECACHE set */
    +
    +#define VFS_CACHE_LIMIT 0x400 /* limit VFS cache page */

    /*
    * Return 1 if free pages are above 'mark'. This takes into account the order
    @@ -910,6 +913,10 @@ int zone_watermark_ok(struct zone *z, in
    if (alloc_flags & ALLOC_HARDER)
    min -= min / 4;

    + if (alloc_flags & ALLOC_PAGECACHE){
    + min = min + VFS_CACHE_LIMIT;
    + }
    +
    if (free_pages <= min + z->lowmem_reserve[classzone_idx])
    return 0;
    for (o = 0; o < order; o++) {
    @@ -1000,8 +1007,12 @@ restart:
    return NULL;
    }

    - page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, order,
    - zonelist, ALLOC_WMARK_LOW|ALLOC_CPUSET);
    + if (gfp_mask & __GFP_PAGECACHE)
    + page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, order,
    + zonelist, ALLOC_WMARK_LOW|ALLOC_CPUSET|ALLOC_PAGECACHE);
    + else
    + page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, order,
    + zonelist, ALLOC_WMARK_LOW|ALLOC_CPUSET);
    if (page)
    goto got_pg;

    @@ -1027,6 +1038,9 @@ restart:
    if (wait)
    alloc_flags |= ALLOC_CPUSET;

    + if (gfp_mask & __GFP_PAGECACHE)
    + alloc_flags |= ALLOC_PAGECACHE;
    +
    /*
    * Go through the zonelist again. Let __GFP_HIGH and allocations
    * coming from realtime tasks go deeper into reserves.
    Index: include/linux/gfp.h
    ===================================================================
    --- include/linux/gfp.h (revision 2645)
    +++ include/linux/gfp.h (working copy)
    @@ -46,6 +46,7 @@ struct vm_area_struct;
    #define __GFP_NOMEMALLOC ((__force gfp_t)0x10000u) /* Don't use
    emergency reserves */
    #define __GFP_HARDWALL ((__force gfp_t)0x20000u) /* Enforce
    hardwall cpuset memory allocs */
    #define __GFP_THISNODE ((__force gfp_t)0x40000u)/* No fallback, no policies */
    +#define __GFP_PAGECACHE ((__force gfp_t)0x80000u) /* Is page cache
    allocation ? */

    #define __GFP_BITS_SHIFT 20 /* Room for 20 __GFP_FOO bits */
    #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
    Index: include/linux/pagemap.h
    ===================================================================
    --- include/linux/pagemap.h (revision 2645)
    +++ include/linux/pagemap.h (working copy)
    @@ -62,7 +62,7 @@ static inline struct page *__page_cache_

    static inline struct page *page_cache_alloc(struct address_space *x)
    {
    - return __page_cache_alloc(mapping_gfp_mask(x));
    + return __page_cache_alloc(mapping_gfp_mask(x)|__GFP_PAGECACHE);
    }

    static inline struct page *page_cache_alloc_cold(struct address_space *x)
    =====================================================
    Welcome any comments and suggestions,

    Thanks,
    -Aubrey
    Index: mm/page_alloc.c
    ===================================================================
    --- mm/page_alloc.c (revision 2645)
    +++ mm/page_alloc.c (working copy)
    @@ -892,6 +892,9 @@ failed:
    #define ALLOC_HARDER 0x10 /* try to alloc harder */
    #define ALLOC_HIGH 0x20 /* __GFP_HIGH set */
    #define ALLOC_CPUSET 0x40 /* check for correct cpuset */
    +#define ALLOC_PAGECACHE 0x80 /* __GFP_PAGECACHE set */
    +
    +#define VFS_CACHE_LIMIT 0x400 /* limit VFS cache page */

    /*
    * Return 1 if free pages are above 'mark'. This takes into account the order
    @@ -910,6 +913,10 @@ int zone_watermark_ok(struct zone *z, in
    if (alloc_flags & ALLOC_HARDER)
    min -= min / 4;

    + if (alloc_flags & ALLOC_PAGECACHE){
    + min = min + VFS_CACHE_LIMIT;
    + }
    +
    if (free_pages <= min + z->lowmem_reserve[classzone_idx])
    return 0;
    for (o = 0; o < order; o++) {
    @@ -1000,8 +1007,12 @@ restart:
    return NULL;
    }

    - page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, order,
    - zonelist, ALLOC_WMARK_LOW|ALLOC_CPUSET);
    + if (gfp_mask & __GFP_PAGECACHE)
    + page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, order,
    + zonelist, ALLOC_WMARK_LOW|ALLOC_CPUSET|ALLOC_PAGECACHE);
    + else
    + page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, order,
    + zonelist, ALLOC_WMARK_LOW|ALLOC_CPUSET);
    if (page)
    goto got_pg;

    @@ -1027,6 +1038,9 @@ restart:
    if (wait)
    alloc_flags |= ALLOC_CPUSET;

    + if (gfp_mask & __GFP_PAGECACHE)
    + alloc_flags |= ALLOC_PAGECACHE;
    +
    /*
    * Go through the zonelist again. Let __GFP_HIGH and allocations
    * coming from realtime tasks go deeper into reserves.
    Index: include/linux/gfp.h
    ===================================================================
    --- include/linux/gfp.h (revision 2645)
    +++ include/linux/gfp.h (working copy)
    @@ -46,6 +46,7 @@ struct vm_area_struct;
    #define __GFP_NOMEMALLOC ((__force gfp_t)0x10000u) /* Don't use emergency reserves */
    #define __GFP_HARDWALL ((__force gfp_t)0x20000u) /* Enforce hardwall cpuset memory allocs */
    #define __GFP_THISNODE ((__force gfp_t)0x40000u)/* No fallback, no policies */
    +#define __GFP_PAGECACHE ((__force gfp_t)0x80000u) /* Is page cache allocation ? */

    #define __GFP_BITS_SHIFT 20 /* Room for 20 __GFP_FOO bits */
    #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
    Index: include/linux/pagemap.h
    ===================================================================
    --- include/linux/pagemap.h (revision 2645)
    +++ include/linux/pagemap.h (working copy)
    @@ -62,7 +62,7 @@ static inline struct page *__page_cache_

    static inline struct page *page_cache_alloc(struct address_space *x)
    {
    - return __page_cache_alloc(mapping_gfp_mask(x));
    + return __page_cache_alloc(mapping_gfp_mask(x)|__GFP_PAGECACHE);
    }

    static inline struct page *page_cache_alloc_cold(struct address_space *x)
    \
     
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