Hugh Dickins <[email protected]> wrote:
>
> The split ptlock patch enlarged the default SMP PREEMPT struct page from
> 32 to 36 bytes on most 32-bit platforms, from 32 to 44 bytes on PA-RISC
> 7xxx (without PREEMPT). That was not my intention, and I don't believe
> that split ptlock deserves any such slice of the user's memory.
Only with preempt and >= 4 CPUs. Vendors don't ship preemptible kernels,
especially on SMP. So the impact is minor.
> While leaving most of the page_private() mods in place for the moment,
> could we please try this patch, or something like it? Again to overlay
> the spinlock_t from &page->private onwards, with corrected BUILD_BUG_ON
> that we don't go beyond ->lru; with poisoning of the fields overlaid,
> and unsplit config verifying that the split config is safe to use them.
Does your family know you do this sort of thing?
> --- 2.6.14-git6/include/linux/mm.h 2005-11-03 18:38:01.000000000 +0000
> +++ linux/include/linux/mm.h 2005-11-03 18:46:06.000000000 +0000
> @@ -226,18 +226,19 @@ struct page {
> * to show when page is mapped
> * & limit reverse map searches.
> */
> - union {
> - unsigned long private; /* Mapping-private opaque data:
> + unsigned long private; /* Mapping-private opaque data:
> * usually used for buffer_heads
> * if PagePrivate set; used for
> * swp_entry_t if PageSwapCache
> * When page is free, this indicates
> * order in the buddy system.
> */
> -#if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
> - spinlock_t ptl;
> -#endif
> - } u;
> + /*
> + * Along with private, the mapping, index and lru fields of a
> + * page table page's struct page may be overlaid by a spinlock
> + * for pte locking: see comment on "split ptlock" below. Please
> + * do not rearrange these fields without considering that usage.
> + */
> struct address_space *mapping; /* If low bit clear, points to
> * inode address_space, or NULL.
> * If page mapped as anonymous
What happened to my suggestion that we use anonymous structs here, and
abandon gcc-2.9x?
> @@ -265,8 +266,8 @@ struct page {
> #endif /* WANT_PAGE_VIRTUAL */
> };
>
> -#define page_private(page) ((page)->u.private)
> -#define set_page_private(page, v) ((page)->u.private = (v))
> +#define page_private(page) ((page)->private)
> +#define set_page_private(page, v) ((page)->private = (v))
Need to rename ->private to ->_private here, otherwise people will start
using page->private again.
-
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