From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 1 Aug 2005 12:15:47 +0200 From: Ingo Molnar Subject: Re: [patch 2.6.13-rc4] fix get_user_pages bug Message-ID: <20050801101547.GA5016@elte.hu> References: <20050801032258.A465C180EC0@magilla.sf.frob.com> <42EDDB82.1040900@yahoo.com.au> <20050801091956.GA3950@elte.hu> <42EDEAFE.1090600@yahoo.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <42EDEAFE.1090600@yahoo.com.au> Sender: owner-linux-mm@kvack.org Return-Path: To: Nick Piggin Cc: Robin Holt , Andrew Morton , Linus Torvalds , Roland McGrath , Hugh Dickins , linux-mm@kvack.org, linux-kernel List-ID: * Nick Piggin wrote: > Ingo Molnar wrote: > >* Nick Piggin wrote: > > >>Feedback please, anyone. > > > > > >it looks good to me, but wouldnt it be simpler (in terms of patch and > >architecture impact) to always retry the follow_page() in > >get_user_pages(), in case of a minor fault? The sequence of minor faults > > I believe this can break some things. Hugh posted an example in his > recent post to linux-mm (ptrace setting a breakpoint in read-only > text). I think? Hugh's posting said: "it's trying to avoid an endless loop of finding the pte not writable when ptrace is modifying a page which the user is currently protected against writing to (setting a breakpoint in readonly text, perhaps?)" i'm wondering, why should that case generate an infinite fault? The first write access should copy the shared-library page into a private page and map it into the task's MM, writable. If this make-writable operation races with a read access then we return a minor fault and the page is still readonly, but retrying the write should then break up the COW protection and generate a writable page, and a subsequent follow_page() success. If the page cannot be made writable, shouldnt the vma flags reflect this fact by not having the VM_MAYWRITE flag, and hence get_user_pages() should have returned with -EFAULT earlier? in other words, can a named MAP_PRIVATE vma with VM_MAYWRITE set ever be non-COW-break-able and thus have the potential to induce an infinite loop? Ingo -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org