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Wong" Cc: "Zhou, Yun" , Christoph Hellwig , cem@kernel.org, linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm Subject: Re: [PATCH] xfs: restore nofs context unconditionally in xfs_trans_roll Message-ID: References: <20260713035505.1635191-1-yun.zhou@windriver.com> <7a053113-f678-486e-a78b-8f5e94e73acb@windriver.com> <20260713230420.GN7195@frogsfrogsfrogs> <0792c175-6a7c-4108-966d-4afbc8c515a2@windriver.com> <20260714175528.GG7380@frogsfrogsfrogs> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260714175528.GG7380@frogsfrogsfrogs> On Tue, Jul 14, 2026 at 10:55:28AM -0700, Darrick J. Wong wrote: > [add linux-mm since we're talking about memalloc_nofs_save] Thanks! > On Tue, Jul 14, 2026 at 10:15:53AM +0800, Zhou, Yun wrote: > > On 7/14/26 07:04, Darrick J. Wong wrote: > > > On Mon, Jul 13, 2026 at 06:28:38AM -0700, Christoph Hellwig wrote: > > > > On Mon, Jul 13, 2026 at 06:06:38PM +0800, Zhou, Yun wrote: > > > > > On 7/13/26 17:09, Christoph Hellwig wrote: > > > > > > On Mon, Jul 13, 2026 at 11:55:05AM +0800, Yun Zhou wrote: > > > > > > > diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c > > > > > > > index 7bfbd9f6f0df..1b36cf12d4e3 100644 > > > > > > > --- a/fs/xfs/xfs_trans.c > > > > > > > +++ b/fs/xfs/xfs_trans.c > > > > > > > @@ -1029,6 +1029,15 @@ xfs_trans_roll( > > > > > > > * duplicate transaction that gets returned. > > > > > > > */ > > > > > > > error = __xfs_trans_commit(tp, true); > > > > > > > + > > > > > > > + tp = *tpp; > > > > > > > + /* > > > > > > > + * __xfs_trans_commit cleared the NOFS flag by calling into > > > > > > > + * xfs_trans_free. Set it again here before doing memory > > > > > > > + * allocations. > > > > > > > + */ > > > > > > > + xfs_trans_set_context(tp); > > > > > > > > > > > > The tp assignment above now returns the incorrect transaction when > > > > > > __xfs_trans_commit fails, so you can't do this. > > > > > > > > > > > > Otherwise yes, this call should move up. I don't really see how > > > > > > it fixes the syzbot report, though. > > > > > > > > > > Thank you very much for your reply. The tp here is a local variable only > > > > > used for convenience within the function. The caller always gets the new > > > > > transaction through *tpp, which was set by xfs_trans_dup() before the commit > > > > > call. Moving tp = *tpp before the error check doesn't change what the caller > > > > > sees - *tpp still points to the new (dup'd) transaction regardless. > > > > > > > > Ah, right. Tis should be fine: > > > > > > > > Reviewed-by: Christoph Hellwig > > > > > > Why not move tp_pflags to the new transaction in xfs_trans_dup like we > > > do for the deferred item list: > > > > > > /* move deferred ops over to the new tp */ > > > xfs_defer_move(ntp, tp); > > > > > > ntp->t_pflags = tp->t_pflags; > > > tp->t_pflags = 0; > > > > That's what the old xfs_trans_switch_context() did before a1ca658d649a > > removed it. The problem is that setting tp->t_pflags = 0 means > > xfs_trans_free() calls memalloc_nofs_restore(0), which relies on that being > > a no-op — an mm implementation detail. A fresh memalloc_nofs_save() on the > > new tp keeps the save/restore pairing correct unconditionally. > > So add a new helper. > > /** > * memalloc_flags_take - move an implicit __GFP_MEMALLOC scope from one > * tracking structure to another. > */ > static inline unsigned int > memalloc_flags_take(unsigned int *old_flags) > { > unsigned int ret = *old_flags; > > *old_flags = 0; > return ret; > } > > and then: > > /* move deferred ops over to the new tp */ > xfs_defer_move(ntp, tp); > > ntp->t_pflags = memalloc_flags_take(&tp->t_pflags); I might stick with the 'move' wording? ie memalloc_flags_move(). And I think you should bury the call to memalloc_flags_move() inside xfs_defer_move(). I don't think there's a case where you'd want to move from one transaction to another without preserving the nofs state, is there? Bit hard to tell since there's only one caller of xfs_defer_move() in the XFS code base.