Messages in this thread | | | Date | Tue, 15 Nov 2005 10:49:16 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH 01/15] mm: poison struct page for ptlock |
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Andrew Morton <akpm@osdl.org> wrote: > > Hugh Dickins <hugh@veritas.com> 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. > > > > 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 too far; with poisoning of the fields overlaid, and > > unsplit SMP config verifying that the split config is safe to use them. > > > > The previous attempt at this patch broke ppc64, which uses slab for its > > page tables - and slab saves vital info in page->lru: but no config of > > spinlock_t needs to overwrite lru on 64-bit anyway; and the only 32-bit > > user of slab for page tables is arm26, which is never SMP i.e. all the > > problems came from the "safety" checks, not from what's actually needed. > > So previous checks refined with #ifdefs, and a BUG_ON(PageSlab) added. > > > > This overlaying is unlikely to be portable forever: but the added checks > > should warn developers when it's going to break, long before any users. > > argh. > > Really, I'd prefer to abandon gcc-2.95.x rather than this. Look: > > struct a > { > union { > struct { > int b; > int c; > }; > int d; > }; > } z; > > main() > { > z.b = 1; > z.d = 1; > } > > It does everything we want.
It occurs to me that we can do the above if (__GNUC__ > 2), or whatever.
That way, the only people who have a 4-byte-larger pageframe are those who use CONFIG_PREEMPT, NR_CPUS>=4 and gcc-2.x.y. An acceptably small community, I suspect.
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