From: Julian Anastasov > Sent: 06 December 2024 16:23 ... > I'm not sure how much memory we can see in small system, > IMHO, problem should not be possible in practice: > > - nobody expects 0 from totalram_pages() in the code > > - order_base_2(sizeof(struct ip_vs_conn)) is probably 8 on 32-bit It is 0x120 bytes on 64bit, so 8 could well be right. > - PAGE_SHIFT: 12 (for 4KB) or more? > > So, if totalram_pages() returns below 128 pages (4KB each) > max_avail will be below 19 (7 + 12), then 19 is reduced with 2 + 1 > and becomes 16, finally with 8 (from the 2nd order_base_2) to reach > 16-8=8. You need a system with less than 512KB (19 bits) to trigger > problem in clamp() that will lead to max below 8. Which pretty much won't happen, I think my (dead) sun3 has more than that. > Further, without > checks, for ip_vs_conn_tab_bits=1 we need totalram_pages() to return 0 > pages. > > > > > Detected by compile time checks added to clamp(), specifically: > > > > minmax.h: use BUILD_BUG_ON_MSG() for the lo < hi test in clamp() > > > > > > Existing or new check? Does it happen that max_avail > > > is a constant, so that a compile check triggers? > > > > Is all stems from order_base_2(totalram_pages()). > > order_base_2(n) is 'n > 1 ? ilog2(n - 1) + 1 : 0'. > > And the compiler generates two copies of the code that follows > > for the 'constant zero' and ilog2() values. > > And the 'zero' case compiles clamp(20, 8, 0) which is errored. > > Note that it is only executed if totalram_pages() is zero, > > but it is always compiled 'just in case'. > > I'm confused with these compiler issues, The compiler is just doing its job. Consider this expression: (x >= 1 ? 2 * x : 1) - 1 It is likely to get converted to: (x >= 1 ? 2 * x - 1 : 0) to avoid the subtract when x < 1. The same thing is happening here. order_base_2() has a (condition ? fn() : 0) in it. All the +/- constants get moved inside, on 64bit that is +12 -2 -1 -9 = 0. Then the clamp() with constants gets moved inside: (condition ? clamp(27, 8, fn() + 0) : clamp(27, 8, 0 + 0)) Now, at runtime, we know that 'condition' is true and (fn() >= 8) so the first clamp() is valid and the second one never used. But this isn't known by the compiler and clamp() detects the invalid call and generates a warning. > if you > think we should go with the patch just decide if it is a > net or net-next material. Your change is safer for bad > max_avail values but I don't expect to see problem while > running without the change, except the building bugs. > > Also, please use nf/nf-next tag to avoid any > confusion with upstreaming... I've copied Andrew M - he's taken the minmax.h change into his mm tree. This is one of the build breakages. It probably only needs to go into next for now (via some route). But I can image the minmax.h changes getting backported a bit. David > > Regards > > -- > Julian Anastasov <ja@xxxxxx> - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
